Mode control of photonic crystal fiber based broadband radiation sources

ABSTRACT

A mode control system and method for controlling an output mode of a broadband radiation source including a photonic crystal fiber (PCF). The mode control system includes at least one detection unit configured to measure one or more parameters of radiation emitted from the broadband radiation source to generate measurement data, and a processing unit configured to evaluate mode purity of the radiation emitted from the broadband radiation source, from the measurement data. Based on the evaluation, the mode control system is configured to generate a control signal for optimization of one or more pump coupling conditions of the broadband radiation source. The one or more pump coupling conditions relate to the coupling of a pump laser beam with respect to a fiber core of the photonic crystal fiber.

This application is a continuation of U.S. patent application Ser. No.17/004,140, filed Aug. 27, 2020, now allowed, which is based upon andclaims the benefit of priority of European patent application no.19194974.2, filed Sep. 2, 2019, of European patent application no.19215183.5, filed Dec. 11, 2019, of European patent application no.20152635.7, filed Jan. 20, 2020, and of European patent application no.20165824.2, filed Mar. 26, 2020, each of the foregoing applications isincorporated herein in its entirety by reference.

FIELD

The present description relates to mode control of photonic crystalfiber based broadband radiation generator, and in particular such abroadband radiation generator in relation to metrology applications inthe manufacture of integrated circuits.

BACKGROUND

A lithographic apparatus is a machine constructed to apply a desiredpattern onto a substrate. A lithographic apparatus can be used, forexample, in the manufacture of integrated circuits (ICs). A lithographicapparatus may, for example, project a pattern (also often referred to as“design layout” or “design”) at a patterning device (e.g., a mask) ontoa layer of radiation-sensitive material (resist) provided on a substrate(e.g., a wafer).

To project a pattern on a substrate a lithographic apparatus may useelectromagnetic radiation. The wavelength of this radiation determinesthe minimum size of features which can be formed on the substrate.Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nmand 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet(EUV) radiation, having a wavelength within the range 4-20 nm, forexample 6.7 nm or 13.5 nm, may be used to form smaller features on asubstrate than a lithographic apparatus which uses, for example,radiation with a wavelength of 193 nm.

Low-k₁ lithography may be used to process features with dimensionssmaller than the classical resolution limit of a lithographic apparatus.In such process, the resolution formula may be expressed as CD=k₁×A/NA,where A is the wavelength of radiation employed, NA is the numericalaperture of the projection optics in the lithographic apparatus, CD isthe “critical dimension” (generally the smallest feature size printed,but in this case half-pitch) and k₁ is an empirical resolution factor.In general, the smaller k₁ the more difficult it becomes to reproducethe pattern on the substrate that resembles the shape and dimensionsplanned by a circuit designer in order to achieve particular electricalfunctionality and performance. To overcome these difficulties,sophisticated fine-tuning steps may be applied to the lithographicprojection apparatus and/or design layout. These include, for example,but not limited to, optimization of NA, customized illumination schemes,use of phase shifting patterning devices, various optimization of thedesign layout such as optical proximity correction (OPC, sometimes alsoreferred to as “optical and process correction”) in the design layout,or other methods generally defined as “resolution enhancementtechniques” (RET). Alternatively, tight control loops for controlling astability of the lithographic apparatus may be used to improvereproduction of the pattern at low k1.

SUMMARY

Metrology tools are used in many aspects of the IC manufacturingprocess, for example as alignment tools for proper positioning of asubstrate prior to an exposure, leveling tools to measure a surfacetopology of the substrate, for e.g., focus control and scatterometrybased tools for inspecting/measuring the exposed and/or etched productin process control. In each case, a radiation (e.g., electromagneticlight) source is often required. For various reasons, includingmeasurement robustness and accuracy, broadband or white light radiationsources are increasingly used for such metrology applications. It wouldbe desirable to improve on present devices for broadband radiation(e.g., white light) generation.

In an aspect, there is provided mode control system, being configuredfor controlling an output mode of a broadband radiation sourcecomprising a photonic crystal fiber (PCF), the mode control systemcomprising: at least one detection unit configured to measure one ormore parameters of radiation emitted from the broadband radiation sourceto generate measurement data; and a processing unit configured toevaluate mode purity of the radiation emitted from the broadbandradiation source, from the measurement data, wherein based on theevaluation, the mode control system is configured to generate a controlsignal for optimization of one or more pump coupling conditions of thebroadband radiation source, the one or more pump coupling conditionsrelating to the coupling of a pump laser beam with respect to a fibercore of the photonic crystal fiber.

In an aspect, there is provided a method of mode control of a broadbandradiation source comprising a photonic crystal fiber, the methodcomprising: measuring one or more parameters of radiation emitted fromthe broadband radiation source to obtain measurement data; evaluatingmode purity of the radiation emitted from the broadband radiationsource, from the measurement data; and generating a control signal tooptimize of one or more pump coupling conditions of the broadbandradiation source, the one or more pump coupling conditions relating tothe coupling of a pump laser beam with respect to a fiber core of thephotonic crystal fiber.

Other aspects comprise a broadband radiation source and metrology devicecomprising a mode control system as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings, in which:

FIG. 1 depicts a schematic overview of a lithographic apparatus;

FIG. 2 depicts a schematic overview of a lithographic cell;

FIG. 3 depicts a schematic representation of holistic lithography,representing a cooperation between three technologies to optimizesemiconductor manufacturing;

FIG. 4 depicts a schematic overview of a scatterometry apparatus used asa metrology device, which may comprise a radiation source according toembodiments of the invention;

FIG. 5 depicts a schematic overview of a level sensor apparatus whichmay comprise a radiation source according to embodiments of theinvention;

FIG. 6 depicts a schematic overview of an alignment sensor apparatuswhich may comprise a radiation source according to embodiments of theinvention;

FIGS. 7A and 7B schematically depict the transverse cross-sections oftwo HC-PCF designs for radiation light generation, including a Kagomedesign as depicted in FIG. 7A and a single-ring design as depicted inFIG. 7B;

FIG. 8 schematically depicts an exemplary gas filled HC-PCF basedbroadband radiation source device;

FIG. 9 is a flowchart describing an operating procedure of a modecontrol system, in accordance with an embodiment of the invention;

FIG. 10 schematically depicts a broadband radiation source equipped witha mode control system, in accordance with an embodiment of theinvention, for optimization and stabilization of the fundamentaltransverse mode LP₀₁ of the radiation source;

FIG. 11 schematically depicts a broadband radiation source equipped witha mode control system, in accordance with an embodiment of theinvention, for optimization and stabilization of the fundamentaltransverse mode LP₀₁ of the radiation source;

FIG. 12 schematically depicts a broadband radiation source equipped witha mode control system, in accordance with an embodiment of theinvention, for optimization and stabilization of the fundamentaltransverse mode LP₀₁ of the radiation source;

FIG. 13 schematically depicts a broadband radiation source equipped witha mode control system, in accordance with an embodiment of theinvention, for optimization and stabilization of the fundamentaltransverse mode LP₀₁ of the radiation source;

FIG. 14 schematically depicts a broadband radiation source equipped witha mode control system, in accordance with an embodiment of theinvention, for optimization and stabilization of the fundamentaltransverse mode LP₀₁ of the radiation source;

FIGS. 15A, 15B and 15C schematically depict a coarse alignmentarrangement according to an embodiment for coarse alignment;

FIG. 16 schematically depicts a coarse alignment arrangement accordingto an embodiment for coarse alignment;

FIG. 17 schematically depicts a coarse alignment arrangement accordingto an embodiment for coarse alignment;

FIG. 18 schematically depicts a specific optical manipulation unitaccording usable in at least the coarse alignment arrangement depictedin FIGS. 15 to 17 ;

FIG. 19 schematically depicts a broadband radiation source equipped witha specific timing control system, in accordance with an embodiment, forcontrol and determination of timing of pulses of the broadband radiationsource; and

FIG. 20 schematically depicts a broadband radiation source equipped witha specific polarization control system, in accordance with anembodiment, for optimization and stabilization of output polarization ofthe broadband radiation source.

DETAILED DESCRIPTION

In the present document, the terms “radiation” and “beam” are used toencompass all types of electromagnetic radiation, including ultravioletradiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) andEUV (extreme ultra-violet radiation, e.g. having a wavelength in therange of about 5-100 nm).

The term “reticle”, “mask” or “patterning device” as employed in thistext may be broadly interpreted as referring to a generic patterningdevice that can be used to endow an incoming radiation beam with apatterned cross-section, corresponding to a pattern that is to becreated in a target portion of the substrate. The term “light valve” canalso be used in this context. Besides the classic mask (transmissive orreflective, binary, phase-shifting, hybrid, etc.), examples of othersuch patterning devices include a programmable mirror array and aprogrammable LCD array.

FIG. 1 schematically depicts a lithographic apparatus LA. Thelithographic apparatus LA includes an illumination system (also referredto as illuminator) IL configured to condition a radiation beam B (e.g.,UV radiation, DUV radiation or EUV radiation), a mask support (e.g., amask table) MT constructed to support a patterning device (e.g., a mask)MA and connected to a first positioner PM configured to accuratelyposition the patterning device MA in accordance with certain parameters,a substrate support (e.g., a wafer table) WT constructed to hold asubstrate (e.g., a resist coated wafer) W and connected to a secondpositioner PW configured to accurately position the substrate support inaccordance with certain parameters, and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., comprising one or more dies) of the substrate W.

In operation, the illumination system IL receives a radiation beam froma radiation source SO, e.g. via a beam delivery system BD. Theillumination system IL may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, and/or other types of optical components, or anycombination thereof, for directing, shaping, and/or controllingradiation. The illuminator IL may be used to condition the radiationbeam B to have a desired spatial and angular intensity distribution inits cross section at a plane of the patterning device MA.

The term “projection system” PS used herein should be broadlyinterpreted as encompassing various types of projection system,including refractive, reflective, catadioptric, anamorphic, magnetic,electromagnetic and/or electrostatic optical systems, or any combinationthereof, as appropriate for the exposure radiation being used, and/orfor other factors such as the use of an immersion liquid or the use of avacuum. Any use of the term “projection lens” herein may be consideredas synonymous with the more general term “projection system” PS.

The lithographic apparatus LA may be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system PS and the substrate W—which is also referred to asimmersion lithography. More information on immersion techniques is givenin U.S. Pat. No. 6,952,253, which is incorporated herein in its entiretyby reference.

The lithographic apparatus LA may also be of a type having two or moresubstrate supports WT (also named “dual stage”). In such “multiplestage” machine, the substrate supports WT may be used in parallel,and/or steps in preparation of a subsequent exposure of the substrate Wmay be carried out on the substrate W located on one of the substratesupport WT while another substrate W on the other substrate support WTis being used for exposing a pattern on the other substrate W.

In addition to the substrate support WT, the lithographic apparatus LAmay comprise a measurement stage. The measurement stage is arranged tohold a sensor and/or a cleaning device. The sensor may be arranged tomeasure a property of the projection system PS or a property of theradiation beam B. The measurement stage may hold multiple sensors. Thecleaning device may be arranged to clean part of the lithographicapparatus, for example a part of the projection system PS or a part of asystem that provides the immersion liquid. The measurement stage maymove beneath the projection system PS when the substrate support WT isaway from the projection system PS.

In operation, the radiation beam B is incident on the patterning device,e.g. mask, MA which is held on the mask support MT, and is patterned bythe pattern (design layout) present on patterning device MA. Havingtraversed the mask MA, the radiation beam B passes through theprojection system PS, which focuses the beam onto a target portion C ofthe substrate W. With the aid of the second positioner PW and a positionmeasurement system IF, the substrate support WT can be moved accurately,e.g., so as to position different target portions C in the path of theradiation beam B at a focused and aligned position. Similarly, the firstpositioner PM and possibly another position sensor (which is notexplicitly depicted in FIG. 1 ) may be used to accurately position thepatterning device MA with respect to the path of the radiation beam B.Patterning device MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks P1, P2 as illustrated occupy dedicated targetportions, they may be located in spaces between target portions.Substrate alignment marks P1, P2 are known as scribe-lane alignmentmarks when these are located between the target portions C.

As shown in FIG. 2 the lithographic apparatus LA may form part of alithographic cell LC, also sometimes referred to as a lithocell or(litho)cluster, which often also includes apparatus to perform pre- andpost-exposure processes on a substrate W. Conventionally these includespin coaters SC to deposit resist layers, developers DE to developexposed resist, chill plates CH and bake plates BK, e.g. forconditioning the temperature of substrates W e.g. for conditioningsolvents in the resist layers. A substrate handler, or robot, RO picksup substrates W from input/output ports I/O1, I/O2, moves them betweenthe different process apparatus and delivers the substrates W to theloading bay LB of the lithographic apparatus LA. The devices in thelithocell, which are often also collectively referred to as the track,are typically under the control of a track control unit TCU that initself may be controlled by a supervisory control system SCS, which mayalso control the lithographic apparatus LA, e.g. via lithography controlunit LACU.

In order for the substrates W exposed by the lithographic apparatus LAto be exposed correctly and consistently, it is desirable to inspectsubstrates to measure properties of patterned structures, such asoverlay errors between subsequent layers, line thicknesses, criticaldimensions (CD), etc. For this purpose, inspection tools (not shown) maybe included in the lithocell LC. If errors are detected, adjustments,for example, may be made to exposures of subsequent substrates or toother processing steps that are to be performed on the substrates W,especially if the inspection is done before other substrates W of thesame batch or lot are still to be exposed or processed.

An inspection apparatus, which may also be referred to as a metrologyapparatus, is used to determine properties of the substrates W, and inparticular, how properties of different substrates W vary or howproperties associated with different layers of the same substrate W varyfrom layer to layer. The inspection apparatus may alternatively beconstructed to identify defects on the substrate W and may, for example,be part of the lithocell LC, or may be integrated into the lithographicapparatus LA, or may even be a stand-alone device. The inspectionapparatus may measure the properties on a latent image (image in aresist layer after the exposure), or on a semi-latent image (image in aresist layer after a post-exposure bake step PEB), or on a developedresist image (in which the exposed or unexposed parts of the resist havebeen removed), or even on an etched image (after a pattern transfer stepsuch as etching).

Typically the patterning process in a lithographic apparatus LA is oneof the most critical steps in the processing which requires highaccuracy of dimensioning and placement of structures on the substrate W.To ensure this high accuracy, three systems may be combined in a socalled “holistic” control environment as schematically depicted in FIG.3 . One of these systems is the lithographic apparatus LA which is(virtually) connected to a metrology tool MT (a second system) and to acomputer system CL (a third system). A goal of such a “holistic”environment is to optimize the cooperation between these three systemsto enhance the overall process window and provide tight control loops tohelp ensure that the patterning performed by the lithographic apparatusLA stays within a process window. The process window defines a range ofprocess parameters (e.g. dose, focus, overlay) within which a specificmanufacturing process yields a defined result (e.g. a functionalsemiconductor device)—typically within which the process parameters inthe lithographic process or patterning process are allowed to vary.

The computer system CL may use (part of) the design layout to bepatterned to predict which resolution enhancement techniques to use andto perform computational lithography simulations and calculations todetermine which mask layout and lithographic apparatus settings achievethe largest overall process window of the patterning process (depictedin FIG. 3 by the double arrow in the first scale SC1). Typically, theresolution enhancement techniques are arranged to match the patterningpossibilities of the lithographic apparatus LA. The computer system CLmay also be used to detect where within the process window thelithographic apparatus LA is currently operating (e.g. using input fromthe metrology tool MT) to predict whether defects may be present due toe.g. sub-optimal processing (depicted in FIG. 3 by the arrow pointing“0” in the second scale SC2).

The metrology tool MT may provide input to the computer system CL toenable accurate simulations and predictions, and may provide feedback tothe lithographic apparatus LA to identify possible drifts, e.g. in acalibration status of the lithographic apparatus LA (depicted in FIG. 3by the multiple arrows in the third scale SC3).

In lithographic processes, it is desirable to make frequentlymeasurements of the structures created, e.g., for process control andverification. Tools to make such measurement are typically calledmetrology tools MT. Different types of metrology tools MT for makingsuch measurements are known, including scanning electron microscopes orvarious forms of scatterometer metrology tools MT. Scatterometers areversatile instruments which allow measurements of the parameters of alithographic process by having a sensor in the pupil or a conjugateplane with the pupil of the objective of the scatterometer, measurementsusually referred as pupil based measurements, or by having the sensor inthe image plane or a plane conjugate with the image plane, in which casethe measurements are usually referred as image or field basedmeasurements. Such scatterometers and the associated measurementtechniques are further described in U.S. patent application publicationnos. US20100328655, US2011102753, US20120044470, US20110249244,US20110026032 and European patent application publication no.EP1,628,164, each of the foregoing patent application publications isincorporated herein in its entirety by reference. Aforementionedscatterometers may measure gratings using radiation from soft x-ray andvisible to near-IR wavelength range.

In a first embodiment, the scatterometer MT is an angular resolvedscatterometer. In such a scatterometer reconstruction methods may beapplied to the measured signal to reconstruct or calculate properties ofthe grating. Such reconstruction may, for example, result fromsimulating interaction of scattered radiation with a mathematical modelof the target structure and comparing the simulation results with thoseof a measurement. Parameters of the mathematical model are adjusteduntil the simulated interaction produces a diffraction pattern similarto that observed from the real target.

In a second embodiment, the scatterometer MT is a spectroscopicscatterometer MT. In such spectroscopic scatterometer MT, the radiationemitted by a radiation source is directed onto the target and thereflected or scattered radiation from the target is directed to aspectrometer detector, which measures a spectrum (i.e. a measurement ofintensity as a function of wavelength) of the specular reflectedradiation. From this data, the structure or profile of the target givingrise to the detected spectrum may be reconstructed, e.g. by RigorousCoupled Wave Analysis and non-linear regression or by comparison with alibrary of simulated spectra.

In a third embodiment, the scatterometer MT is an ellipsometricscatterometer. The ellipsometric scatterometer allows for determiningparameters of a lithographic process by measuring scattered radiationfor each polarization state. Such metrology apparatus emits polarizedradiation (such as linear, circular, or elliptic) by using, for example,appropriate polarization filters in the illumination section of themetrology apparatus. A source suitable for the metrology apparatus mayprovide polarized radiation as well. Various embodiments of existingellipsometric scatterometers are described in U.S. patent applicationpublication nos. 2007-0296960, 2008-0198380, 2009-0168062, 2010-0007863,2011-0032500, 2011-0102793, 2011-0188020, 2012-0044495, 2013-0162996 and2013-0308142, each of which is incorporated herein in its entirety byreference.

A metrology apparatus, such as a scatterometer, is depicted in FIG. 4 .It comprises a broadband (white light) radiation projector 2 whichprojects radiation onto a substrate W. The reflected or scatteredradiation is passed to a spectrometer detector 4, which measures aspectrum 6 (i.e. a measurement of intensity as a function of wavelength)of the specular reflected radiation. From this data, the structure orprofile 8 giving rise to the detected spectrum may be reconstructed byprocessing unit PU, e.g. by Rigorous Coupled Wave Analysis andnon-linear regression or by comparison with a library of simulatedspectra as shown at the bottom of FIG. 3 . In general, for thereconstruction, the general form of the structure is known and someparameters are assumed from knowledge of the process by which thestructure was made, leaving only a few parameters of the structure to bedetermined from the scatterometry data. Such a scatterometer may beconfigured as a normal-incidence scatterometer or an oblique-incidencescatterometer.

Overall measurement quality of a lithographic parameter via measurementof a metrology target is at least partially determined by themeasurement recipe used to measure this lithographic parameter. The term“substrate measurement recipe” may include one or more parameters of themeasurement itself, one or more parameters of the one or more patternsmeasured, or both. For example, if the measurement used in a substratemeasurement recipe is a diffraction-based optical measurement, one ormore of the parameters of the measurement may include the wavelength ofthe radiation, the polarization of the radiation, the incident angle ofradiation relative to the substrate, the orientation of radiationrelative to a pattern on the substrate, etc. One of the criteria toselect a measurement recipe may, for example, be a sensitivity of one ofthe measurement parameters to processing variations. More examples aredescribed in U.S. patent application publication nos. US 2016-0161863and US 2016/0370717, each of which is incorporated herein in itsentirety by reference.

Another type of metrology tool used in IC manufacture is a topographymeasurement system, level sensor or height sensor. Such a tool may beintegrated in the lithographic apparatus, for measuring a topography ofa top surface of a substrate (or wafer). A map of the topography of thesubstrate, also referred to as height map, may be generated from thesemeasurements indicating a height of the substrate as a function of theposition on the substrate. This height map may subsequently be used tocorrect the position of the substrate during transfer of the pattern onthe substrate, in order to provide an aerial image of the patterningdevice in a properly focus position on the substrate. It will beunderstood that “height” in this context refers to a dimension broadlyout of the plane to the substrate (also referred to as Z-axis).Typically, the level or height sensor performs measurements at a fixedlocation (relative to its own optical system) and a relative movementbetween the substrate and the optical system of the level or heightsensor results in height measurements at locations across the substrate.

An example of a level or height sensor LS is schematically shown in FIG.5 , which illustrates only the principles of operation. In this example,the level sensor comprises an optical system, which includes aprojection unit LSP and a detection unit LSD. The projection unit LSPcomprises a radiation source LSO providing a beam of radiation LSB whichis incident on a projection grating PGR of the projection unit LSP. Theradiation source LSO may be, for example, a narrowband or broadbandradiation source, such as a supercontinuum radiation source, polarizedor non-polarized, pulsed or continuous, such as a polarized ornon-polarized laser beam. The radiation source LSO may include aplurality of radiation sources having different colors, or wavelengthranges, such as a plurality of LEDs. The radiation source LSO of thelevel sensor LS is not restricted to visible radiation, but mayadditionally or alternatively encompass UV and/or IR radiation and anyrange of wavelengths suitable to reflect from a surface of a substrate.

The projection grating PGR is a periodic grating comprising a periodicstructure resulting in a beam of radiation BE1 having a periodicallyvarying intensity. The beam of radiation BE1 with the periodicallyvarying intensity is directed towards a measurement location MLO on asubstrate W having an angle of incidence ANG with respect to an axisperpendicular (Z-axis) to the incident substrate surface between 0degrees and 90 degrees, typically between 70 degrees and 80 degrees. Atthe measurement location MLO, the patterned beam of radiation BE1 isredirected by the substrate W (indicated by arrows BE2) and directedtowards the detection unit LSD.

In order to determine the height level at the measurement location MLO,the level sensor further comprises a detection system comprising adetection grating DGR, a detector DET and a processing unit (not shown)for processing an output signal of the detector DET. The detectiongrating DGR may be identical to the projection grating PGR. The detectorDET produces a detector output signal indicative of the radiationreceived, for example indicative of the intensity of the radiationreceived, such as a photodetector, or representative of a spatialdistribution of the intensity received, such as a camera. The detectorDET may comprise any combination of one or more detector types.

By means of triangulation techniques, the height level at themeasurement location MLO can be determined. The detected height level istypically related to the signal strength as measured by the detectorDET, the signal strength having a periodicity that depends, amongstothers, on the design of the projection grating PGR and the (oblique)angle of incidence ANG.

The projection unit LSP and/or the detection unit LSD may includefurther optical elements, such as lenses and/or mirrors, along the pathof the patterned beam of radiation between the projection grating PGRand the detection grating DGR (not shown).

In an embodiment, the detection grating DGR may be omitted, and thedetector DET may be placed at the position where the detection gratingDGR is located. Such a configuration provides a more direct detection ofthe image of the projection grating PGR.

In order to cover the surface of the substrate W effectively, a levelsensor LS may be configured to project an array of reference beams BE1onto the surface of the substrate W, thereby generating an array ofmeasurement areas MLO or spots covering a larger measurement range.

Various height sensors of a general type are disclosed for example inU.S. Pat. Nos. 7,265,364 and 7,646,471, each of which is incorporatedherein in its entirety by reference. A height sensor using UV radiationinstead of visible or infrared radiation is disclosed in U.S. patentapplication publication no. US2010233600, which is incorporated hereinin its entirety by reference. In PCT patent application publication no.WO2016102127A1, which is incorporated herein in its entirety byreference, a compact height sensor is described which uses amulti-element detector to detect and recognize the position of a gratingimage, without needing a detection grating.

Another type of metrology tool used in IC manufacture is an alignmentsensor. A significant aspect of performance of the lithographicapparatus is therefore the ability to place the applied patterncorrectly and accurately in relation to features laid down in previouslayers (by the same apparatus or a different lithographic apparatus).For this purpose, the substrate is provided with one or more sets ofmarks or targets. Each mark is a structure whose position can bemeasured at a later time using a position sensor, typically an opticalposition sensor. The position sensor may be referred to as “alignmentsensor” and marks may be referred to as “alignment marks”.

A lithographic apparatus may include one or more (e.g. a plurality of)alignment sensors by which positions of alignment marks provided on asubstrate can be measured accurately. Alignment (or position) sensorsmay use optical phenomena such as diffraction and interference to obtainposition information from alignment marks formed on the substrate. Anexample of an alignment sensor for a lithographic apparatus is based ona self-referencing interferometer as described in U.S. Pat. No.6,961,116, which is incorporated herein in its entirety by reference.Various enhancements and modifications of the position sensor have beendeveloped, for example as disclosed in U.S. patent applicationpublication no. US 2015-261097, which is incorporated herein in itsentirety by reference.

FIG. 6 is a schematic block diagram of an embodiment of an alignmentsensor AS, such as is described, for example, in U.S. Pat. No.6,961,116, which is incorporated herein in its entirety by reference.Radiation source RSO provides a beam RB of radiation of one or morewavelengths, which is diverted by diverting optics onto a mark, such asmark AM located on substrate W, as an illumination spot SP. In thisexample the diverting optics comprises a spot mirror SM and an objectivelens OL. The illumination spot SP, by which the mark AM is illuminated,may be slightly smaller in width (e.g., diameter) than the width of themark itself.

Radiation diffracted by the alignment mark AM is collimated (in thisexample via the objective lens OL) into an information-carrying beam IB.The term “diffracted” is intended to include zero-order diffraction fromthe mark (which may be referred to as reflection). A self-referencinginterferometer SRI, e.g. of the type disclosed in U.S. Pat. No.6,961,116 mentioned above, interferes the beam IB with itself afterwhich the beam is received by a photodetector PD. Additional optics (notshown) may be included to provide separate beams in case more than onewavelength is created by the radiation source RSO. The photodetector maybe a single element, or it may comprise a number of pixels, if desired.The photodetector may comprise a sensor array.

The diverting optics, which in this example comprises the spot mirrorSM, may also serve to block zero order radiation reflected from themark, so that the information-carrying beam IB comprises only higherorder diffracted radiation from the mark AM (this is not essential tothe measurement, but improves signal to noise ratios).

Intensity signals SI are supplied to a processing unit PU. By acombination of optical processing in the block SRI and computationalprocessing in the unit PU, values for X- and Y-position on the substraterelative to a reference frame are output.

A single measurement of the type illustrated only fixes the position ofthe mark within a certain range corresponding to one pitch of the mark.Coarser measurement techniques are used in conjunction with this toidentify which period of a sine wave is the one containing the markedposition. The same process at coarser and/or finer levels may berepeated at different wavelengths for increased accuracy and/or forrobust detection of the mark irrespective of the materials from whichthe mark is made, and materials on and/or below which the mark isprovided. The wavelengths may be multiplexed and de-multiplexedoptically so as to be processed simultaneously, and/or they may bemultiplexed by time division or frequency division.

In this example, the alignment sensor and spot SP remain stationary,while it is the substrate W that moves. The alignment sensor can thus bemounted rigidly and accurately to a reference frame, while effectivelyscanning the mark AM in a direction opposite to the direction ofmovement of substrate W. The substrate W is controlled in this movementby its mounting on a substrate support and a substrate positioningsystem controlling the movement of the substrate support. A substratesupport position sensor (e.g. an interferometer) measures the positionof the substrate support (not shown). In an embodiment, one or more(alignment) marks are provided on the substrate support. A measurementof the position of the marks provided on the substrate support allowsthe position of the substrate support as determined by the positionsensor to be calibrated (e.g. relative to a frame to which the alignmentsystem is connected). A measurement of the position of the alignmentmarks provided on the substrate allows the position of the substraterelative to the substrate support to be determined.

For optical semiconductor metrology, inspection applications, such as inany of the aforementioned metrology tools, a bright radiation sourcewhich outputs coherent radiation, simultaneously covering a broadwavelength range (e.g., from UV to IR), is often desired. Such abroadband radiation source can help improve the flexibility androbustness of applications by allowing substrates with differentmaterial characteristics to be optically examined in the samesetup/system without a need for any hardware change (e.g., changing aradiation source so as to have a specific wavelength). Allowing thewavelength to be optimized for a specific application also means thatthe accuracy of measurements can be further increased.

Gas lasers, which are based on the gas-discharge effect tosimultaneously emit multiple wavelengths, can be used in theseapplications. However, intrinsic issues such as high intensityinstability and low spatial incoherence associated with gas lasers canmake them unsuitable. Alternatively, outputs from multiple lasers (e.g.,solid-state lasers) with different wavelengths can be spatially combinedinto the optical path of a metrology or inspection system so as toprovide a multiple wavelength source. The complexity and highimplementation costs, which increases with the number of wavelengthsdesired, prevents such a solution from being widely used. In contrast, afiber-based broadband or white light laser, also called a supercontinuumlaser, is able to emit radiation with high spatial coherence and broadspectral coverage, e.g., from UV to IR, and therefore is a veryattractive and practical option.

A hollow-core photonic crystal fiber (HC-PCF) is a special type ofoptical fiber that comprises a central hollow core region and an innercladding structure surrounding the hollow core, both of which extendaxially along the entire fiber. The radiation guidance mechanism isenabled by the inner cladding waveguide structure, which may comprise,for example, thin-walled glass elements. The radiation is thus confinedpredominantly inside a hollow core and propagates along the fiber in theform of transverse core modes.

A number of types of HC-PCFs can be engineered, each based on adifferent physical guidance mechanism. Two such HC-PCFs include:hollow-core photonic bandgap fibers (HC-PBFs) and hollow-coreanti-resonant reflecting fibers (HC-ARFs).

HC-PCFs comprise hollow channels which are filled with a fluid, suchthat they possess resultant desired characteristics for variousradiation guiding applications; for example, high-power beam deliveryusing HC-PBFs and gas-based white light generation (or supercontinuumgeneration) using HC-ARFs. Detail on the design and manufacture ofHC-PCFs can be found in U.S. patent application publication no.US2004-0175085 (for HC-PBFs) and European patent application publicationno. EP3136143 (for HC-ARFs), each of which is incorporated herein in itsentirety by reference. HC-PBFs are configured to offer low loss butnarrow bandwidth radiation guidance via a photonic bandgap effectestablished by the cladding structure surrounding the central hollowcore. Whereas HC-ARFs are engineered to significantly broaden thetransmission bandwidth via anti-resonant reflection of radiation fromthe cladding.

FIG. 7 depicts in cross-section, two types of HC-ARFs. FIG. 7A shows aKagome fiber, comprising a Kagome lattice structure as its cladding CLAdefining a hollow fiber core FCO. This arrangement may be surrounded byone or more outer coatings OCO. FIG. 7B shows a single-ring or revolverfibers, where the hollow core region FCO is formed and surrounded by alayer of non-touching rings CLA.

For gas-based radiation (e.g., white light) generation, a HC-ARF may becomprised within a gas cell, which is designed to operate, for example,at a pressure up to many 10s of bars (e.g., between 3-100 bar). Agas-filled HC-ARF can act as an optical frequency converter when beingpumped by an ultrashort pump laser pulse with sufficient peak power. Thefrequency conversion from ultrashort pump laser pulses to broadbandlaser pulses is enabled by a complicated interplay of the dispersion andnonlinear optical processes inside the gas-filled fiber. The convertedlaser pulses are predominantly confined within the hollow core in theform of transverse core modes and guided to the fiber end. Transversecore modes that are supported by the fiber can be described as linearpolarized (LP) modes. In the LP notation, an LP mode is referred to asLP_(mn), where m and n subscripts are integers representing theazimuthal and radial order of a particular mode. The fundamental mode isLP₀₁ mode. Part of the radiation, for example higher order transversecore modes or specific wavelengths, may leak from the hollow corethrough the inner cladding waveguide structure and undergoes strongattenuation during its propagation along the fiber. The core region andthe cladding region of a HC-ARF can be configured such that the higherorder core modes are phase matched to the higher order cladding modes.In this way, the higher order core modes can resonantly couple with thehigher order cladding modes which subsequently get attenuated orsuppressed. In such a manner, low loss and effectively single transversemode transmission can be obtained in a broad spectral range.

One or more spatio-temporal transmission characteristics of a laserpulse, e.g. its spectral amplitude and phase, transmitted along a PCF(such as an HC-PCF) can be varied and tuned through adjustment of one ormore pump laser parameters, one or more filling gas parameters, one ormore fiber parameters and/or one or more pump coupling conditions. Theone or more transmission characteristics may include one or moreselected from: output power, output mode profile, output temporalprofile, width of the output temporal profile (or output pulse width),output spectral profile, and/or bandwidth of the output spectral profile(or output spectral bandwidth). The one or more pump laser parametersmay include one or more selected from: pump wavelength, pump pulseenergy, pump pulse width, and/or pump pulse repetition rate. The one ormore fiber parameters may include one or more selected from: fiberlength, size and/or shape of the hollow core, size and/or shape of thecladding structure, and/or thickness of the walls surrounding the hollowcore. The one or more filling gas parameters may include one or moreselected from: gas type, gas pressure and/or gas temperature. The one ormore pump coupling conditions, determining how well a pump laser beam iscoupled into a fiber core, may include one or more selected from:angular offset of the pump laser beam with respect to the fiber core,lateral offset of the pump laser beam with respect to the fiber coreand/or mode matching between the pump laser beam and the fiber core. Themode matching between the pump laser beam and the fiber core may bedetermined by one or more parameters such as beam width (e.g., diameter)of the pump laser beam, divergence of the pump laser beam, width (e.g.,diameter) of the hollow core and/or numerical aperture (NA) of thehollow core.

The filling gas of a HC-PCF can be a noble gas such as helium, neon,argon, krypton, xenon, a Raman active gas such as hydrogen, deuterium,and/or nitrogen, or a gas mixture such as an argon/hydrogen mixture, axenon/deuterium mixture, a krypton/nitrogen mixture, or anitrogen/hydrogen mixture. Depending on the type of filling gas, thenonlinear optical processes can include modulational instability (MI),soliton fission, Kerr effect, Raman effect and/or dispersive wavegeneration, details of which are described in PCT patent applicationpublication no. WO2018/127266A1 and U.S. Pat. No. 9,160,137, each ofwhich is incorporated herein in its entirety by reference. Since thedispersion of the filling gas can be tuned by varying the gas cellpressure, the generated broadband pulse dynamics and the associatedspectral broadening characteristics can be adjusted so as to optimizethe frequency conversion. The generated broadband laser output can coverwavelengths from UV (e.g., <200 nm) to mid-IR (e.g., >2000 nm).

When applied to semiconductor metrology and alignment applications, suchas in any of the aforementioned metrology tools, the transverse mode ofthe output radiation of a HC-PCF based broadband radiation source isdesired to be the fundamental transverse mode, i.e. LP₀₁. In otherwords, a broadband laser beam with a high or maximized mode purity,defined as the ratio between the power in the fundamental transversemode and the total output power, is typically desired. This is due tothe fact that the fundamental transverse mode has a much lowertransmission loss through a PCF (e.g., a HC-PCF) than that of higherorder modes (HOMs). Hence, it is more power efficient if all the pumpradiation is coupled into the fundamental transverse mode of the fiber.Furthermore, the presence of HOMs degrades the mode quality and/orintensity stability of the broadband output. In many applications wherea Gaussian beam profile is desired, a broadband output with a poor modepurity will experience a significant power loss as the HOM content ofthe output will be removed, e.g., by spatial filtering, duringtransmission. The degradation of the intensity stability results in highmeasurement noise and/or poor measurement consistency.

FIG. 8 schematically illustrates an exemplary HC-PCF based broadbandradiation source 800. A collimated pump laser beam 811, comprising atrain of pump pulses at a specific repetition rate, is output from apump laser 810 and used as an input laser beam for generation ofbroadband radiation in the HC-PCF 841. The propagation of the collimatedpump laser beam is controlled by one or more beam steering components(e.g., forming part of a beam delivery system), here depicted as twosteering mirrors 820 and 821 and is directed to pass through a focusinglens 830. The focusing lens creates a suitable focus of the pump laserbeam which is mode matched to the fiber core of the HC-PCF 841. Thefocused pump laser beam transmits through an input optical window 842before being coupled into the core of the HC-PCF 841. The HC-PCF 841,having a specific fiber length, may employ the Kagome design or thesingle-ring design with reference to FIG. 7 . Alternatively, one or moreother fiber designs (not shown) such as a solid core design, aninhibited coupling design, a hypocycloid-core Kagome, and/or a nestedtubular design may be used. In this example, the entire HC-PCF 841 iscomprised in a single pressure-tight gas cell 840 filled with a workinggas or a gas mixture at a specific pressure or with a pressuredistribution. After being coupled into the gas-filled HC-PCF, pump laserpulses propagate along the fiber where they experience significantspectral broadening. Resultant broadband laser pulses 880 aresubsequently discharged from the gas cell 840 via the output opticalwindow 843. The broadband laser beam 880 is then collimated by acollimating lens 831 to a suitable beam size.

To fill the HC-PCF with a working gas, the gas cell may be incommunication with a pressurized gas supply or reservoir (not shown).The inner surfaces of the walls and windows of the gas cell enclose acavity. In an embodiment, the axis of the gas cell is substantiallyparallel to the axis of the HC-PCF.

The pump pulse duration may be chosen to be greater than 10 fs, and morespecifically within the range of: 10 fs to 100 ps, 10 fs to 30 ps or 10fs to 1 ps. The pump wavelength may be chosen from the visible regime,near-IR regime or mid-IR regime. The pump laser pulses may have arepetition frequency of several-hundred hertz (Hz), kilohertz (kHz), ormegahertz (MHz). In particular the repetition rate may be chosen to bein the range of 100 kHz to 100 MHz, such as 100 kHz, 500 kHz, 1 MHz, 5MHz, 10 MHz, 20 MHz, 30 MHz, 40 MHz.

The alignment of the pump laser beam with respect to the HC-PCF maycomprise two main steps, i.e., coarse alignment and fine alignment.Coarse alignment is performed at a sufficiently low pump pulse energy orpump power to prevent damage to the HC-PCF. This step is to help ensurethe pump beam is properly coupled into the hollow core of the HC-PCF andthe transverse core modes are excited at the front (or input) facet ofthe HC-PCF. Without coarse alignment, damage might occur when the centerof the high power pump laser beam hits a cladding wall of the HC-PCF.Once the transverse core modes are excited and transmission efficiency,defined as the ratio between the fiber output power and the fiber inputpower, is maximized at the low power level, fine alignment at a highpower level is started. Again, the purpose of fine alignment is tofurther maximize the transmission efficiency. However, in a conventionaloptimization method, a maximized transmission efficiency (i.e. amaximized output power at a given input power) does not necessarilycorrespond to the highest mode purity. In other words, HOM content canstill be present in the output of a broadband radiation source evenafter the transmission efficiency is optimized. A main reason for thisdiscrepancy is that the transverse beam profile of the pump laser beamis typically imperfect, i.e. M²>1. Hence, no matter how well the overalltransmission efficiency is optimized, HOMs will be excited.

Both coarse alignment and fine alignment (e.g., as part of a pumpcoupling optimization) can be enabled by one or more selected from: 1)movement of at least one beam steering component in a beam deliverysystem (represented as two steering mirrors in the FIG. 8 and otherFigures, although this is a purely exemplary beam delivery system); 2)movement of the gas cell and/or 3) movement of the focusing lens (ifavailable). The one or more beam steering components or steering mirrorsmay be held by one or more kinematic mirror mounts which can be adjustedmanually and/or electrically via one or more actuators such as one ormore piezoelectric actuators. Mirrors may be fixed by any suitablefixing arrangement, such as for example directly adhering a portion oftheir back surfaces to the mirror mounts or by using a setscrew. The gascell may be mounted on a stage module comprising one or more (e.g.,piezo) stages. The stage module may provide movement with multipledegrees of freedom, e.g., six degrees of freedom. Additionally oralternatively, the alignment of the pump laser beam with respect to theHC-PCF can be achieved by inserting one or more extra optical componentsinto the beam path of the pump laser beam (e.g., as part of a beamdelivery system or otherwise). Such one or more optical components maycomprise, for example, two or more (rotatable) optical wedges or any oneor more other optical components that can generate desired movement(angular and/or lateral) of the input pump laser beam. Any one or moreof these alignment strategies and methods may be used in a pump couplingoptimization of methods described herein.

In order to achieve a good mode matching between the pump laser beam andthe fiber core, the pump laser beam may be focused by an optical element(e.g., lens) before entering into the fiber, the lens properties beingsuch that the divergence and the width (e.g., diameter) of the focusedpump laser beam is well matched with the numerical aperture (NA) and themode field width (e.g., diameter) of the fiber core. Since thecharacteristics of the focused pump laser beam are determined by thelens when the input pump laser beam is fixed, a different HC-PCF mayhave different fiber characteristics and therefore may require adifferent focusing lens for an optimal mode matching. Similarly, whencharacteristics of the HC-PCF are fixed, a different pump laser beam,e.g., having a different beam width or beam divergence, may require adifferent focusing lens to keep the focused beam width the same.

The presence of HOMs degrades the mode quality as well as the intensitystability of the broadband output. The inconsistency in mode purityacross the full output spectrum results in wavelength dependent outputperformance, which will negatively impact the reliability andrepeatability of the metrology data should such a broadband radiationsource be used in any of the aforementioned metrology tools. Typically,a HOM is triggered when one or more pump coupling conditions are notoptimized. In some situations where the mode purity of a broadbandradiation source is fully optimized across the full output spectralrange, HOMs can still appear at the output after a certain amount ofruntime. The appearance of HOMs during operation is caused by e.g.,thermal and/or vibrational drifts induced degradation of pump couplingconditions.

Referring back to the exemplary setup of a broadband radiation source inFIG. 8 , a small amount of the incident pump laser beam can leak throughthe reflective surface of a beam steering component such as a steeringmirror and be incident on the component mounting mechanism. This pumpleakage can heat up the mounting mechanism and change its condition. Forexample, this heating can cause softening of an adhesive used to bondthe component to the mounting mechanism, resulting in a smallmisalignment of the component and hence the pump laser beam with respectto the HC-PCF. Due to the fact that HC-PCF based broadband radiationsources are alignment sensitive, a misalignment of the pump laser beamwith respect to the HC-PCF can significantly degrade the couplingcondition of the pump laser beam into the fiber core, resulting in thegeneration of HOMs and/or the degradation of mode purity of thebroadband output. Misalignment and/or a change of alignment of the pumplaser beam could also be induced by one or more other factors, such asambient temperature oscillations or external vibrations. In a similarmanner, thermal and/or vibrational drifts of the HC-PCF can result inthe appearance of HOMs and/or the degradation of mode purity of thebroadband output.

Therefore, there is a desire to optimize and/or stabilize the broadbandoutput, in particular the LP₀₁ mode, during operation of a broadbandradiation source. A mode control method and apparatus is thereforeproposed for addressing the above-mentioned mode purity issue associatedwith a PCF based broadband radiation source.

FIG. 9 illustrates an operating procedure of the proposed mode controlsystem in accordance with an embodiment. At step 910, one or moreparameters of the broadband output beam are measured. Such one or morebeam parameters (i.e., one or more parameters of the broadband outputbeam) are indicative of the output performance in respect of thefundamental mode purity. At step 920, the measured data is processed. Atstep 930, the processed data is evaluated by following a pre-defined setof criteria. The detail of steps 910 to 930 is largely dependent on thebeam parameter(s) being monitored and/or the hardware set-up and moredetailed examples of these steps will be described below. According tothe outcome of such evaluation, a control signal will be generated atstep 940. At step 950, the control signal is used to control one or morecomponents of the broadband radiation source. The control of such one ormore components of the broadband radiation source optimizes pumpcoupling conditions such that the mode purity in terms of thefundamental transverse mode LP₀₁ is increased or maximized.

The optimization of pump coupling conditions can be achieved in variousways. Any method which improves the coupling of the pump laser into thePCF can be used, and can be effected by moving the pump beam withrespect to the PCF (e.g., via beam steering components or otherwise),moving the PCF with respect to the pump beam or moving both incombination; or alternatively or additionally by changing the positionor configuration of any intervening optical component such as a focusingcomponent. This can be performed while monitoring the reference beam(and therefore the output beam) to help ensure mode purity is optimized.As such, the method may be implemented in closed-loop operation suchthat the change of the transverse mode of the broadband output can becontinuously monitored and optimized. Depending on the type of beamparameter monitored, different detection mechanisms may be used, each ofwhich may require one or more different measuring devices or components.

FIG. 10 schematically illustrates a broadband radiation source equippedwith a mode control system 1000, in accordance with an embodiment, foroptimization and stabilization of the fundamental transverse mode LP₀₁of the radiation source. In this embodiment, the broadband radiationsource is essentially similar to the exemplary radiation source 800illustrated in FIG. 8 . For brevity, equivalent components and featuresmay be provided with like reference numerals in remaining Figures afterthe introduced of the reference numeral instead of being individuallydescribed (e.g., labels 811 in FIG. 8, 1011 in FIG. 10, 1111 in FIG. 11, etc. all describe the pump laser beam).

As illustrated in FIG. 10 , part of the main broadband output beam 1080is reflected by the front surface of beam splitter 1051 (whichoptionally may be comprised within the detection unit 1050) and used asa reference beam 1081. It is noted that the beam splitter 1051 shouldnot cause any spatial and spectral distortion to the reference beam,such that reference beam 1081 and the main broadband output beam 1080are considered to share same beam characteristics. The reference beam1081 is used by a mode control system 1000 for optimization andstabilization of the fundamental transverse mode LP₀₁ of the broadbandradiation source. The mode control system 1000 comprises a detectionunit 1050, a processing unit 1060 and a control unit 1070. The detectionunit 1050 measures one or more parameters of the broadband output. Theresultant measurement data is sent to the processing unit 1060 for dataprocessing and evaluation. Based on a result of the evaluation, acontrol signal is generated and used by the control unit 1070 to controlone or more beam control system components accordingly. Such one or morebeam control system components may comprise, for example, one or morebeam delivery or beam steering components (e.g., the steering mirror1020, the steering mirror 1021 or an actuator therefor), an actuator orstage which moves the gas cell 1040, an actuator which moves the(optional) focusing lens 1030 and/or an actuator which alters theabsolute polarization angle (e.g., where the HC-PCF is polarizationmaintaining, and a certain absolute orientation of the polarization ofthe radiation is desired), such as a rotating polarizer. The datameasurement and evaluation may be performed in a continuous orperiodical manner.

In an embodiment, the detection unit comprises a bandpass filter 1052and an illumination measuring device such as power measuring device1053, (e.g., a power meter) arranged such that the power measuringdevice 1053 measures the power of the broadband output in the spectralrange of the passband of the filter. Optionally, the bandpass filter1052 arrangement may have a variable passband arrangement. In this way,a plurality of power values measured in a plurality of spectral rangescan be obtained, each power value corresponding to each spectral rangeof each bandpass filter. This may be effected by mounting the bandpassfilter 1052 on a filter wheel together with one or more differentbandpass filters such that, on completion of a power measurement (whichcorresponds with step 910 of the method of FIG. 9 ) in a particularspectral range, the filter wheel can be rotated so as to enable adifferent bandpass filter. Other arrangements of obtaining variablebandpass characteristics may be envisaged, including providing a movablehigh-pass and movable low-pass continuously variable filter in seriesfor example.

The measured power values from power measuring device 1053 and thespectral information of the bandpass filter(s) used in the measurementsare then sent to the processing unit 1060 to calculate one or morespectral parameter values, such as one or more power spectral density(PSD) or one or more energy spectral density (ESD) values (thiscorresponds with step 920 of the method of FIG. 9 ). The calculated oneor more spectral parameter values are subsequently evaluated (thiscorresponds with step 930 of the method of FIG. 9 ) in the processingunit 1060 by following a certain set of criteria. The processing unit1060 may comprise a processor which is configured to process themeasured data and subsequently perform evaluation on the processed data.Alternatively or in addition, the evaluation may be performed directlyon the measured one or more values.

During data evaluation, the calculated one or more spectral parametervalues may be compared with one or more reference spectral parametervalues in corresponding spectral ranges and, based on the comparison, aset of deviation values generated; where a deviation value comprises ameasure of the degree of deviation of a calculated spectral parametervalue with respect to the reference spectral parameter value. The one ormore reference spectral parameter values may correspond to a mode purityof the broadband output beam indicative of an optimal output. The set ofdeviation values may be evaluated against a set of predefined deviationthresholds for corresponding spectral ranges in order to determinewhether the pump coupling conditions of the broadband radiation sourceare acceptable. The predefined deviation thresholds may be set to apercentage in the range of, for example, 5% to 25% of the referencespectral parameter values (e.g., 5%, 10%, 15% or 20% of the referencespectral parameter values). If the deviation values are indicative ofunacceptable pump coupling conditions such that the mode purity of thebroadband output is sub-optimal, a control signal is generated and/orvaried accordingly (this corresponds with step 940 of the method of FIG.9 ). Based on the control signal, the control unit 1070 will command oneor more components so as to improve/optimize the pump couplingconditions and improve or maximize mode purity of the broadband output(this corresponds with step 950 of the method of FIG. 9 ).

A couple of specific, and purely exemplary, methods for optimization ofpump coupling conditions will now be described, where the control unit1070 (or processing unit 1060) may command two beam steering components(e.g., steering mirrors 1020, 1121) to be incrementally scanned in bothhorizontal and vertical directions. The horizontal direction is definedto be parallel to the optical table plane and the vertical direction isdefined to be perpendicular to the table plane. This scanningimplementation is applicable to all embodiments described herein,although the beam parameter(s) being monitored may be different forlater embodiments (as will be apparent).

In a first such specific scanning implementation, this scanning may beperformed by scanning a first of the mirrors (e.g., mirror 1020) in ameandering or zig-zag path: e.g., it may be commanded to scanrepetitions of the following sequence: a first incremental scan in adesired range in the x direction and a single movement in the ydirection, repeated to cover a desired range in the y direction. Whilethe first mirror 1020 is scanning, the second mirror 1021 is maintainedin its original position. After each incremental movement, a measurementis taken, e.g., a power measurement. In such a manner, a power mapand/or a calculated PSD map in a predefined area is generated. Based onthe map(s), the position of the first mirror is optimized and the secondmirror, e.g., mirror 1021, will start scanning in the same manner. Atthe end of the mirror scanning, the position of the second mirror willalso be optimized.

In the above-mentioned scanning routine, the two mirrors are decoupled.When one mirror is scanning, the other one is assumed to be maintainedin an optimal position. Such an arrangement is therefore not ideal whenboth mirrors have drifted from their respective optimal positions. Assuch, in a second scanning implementation, a co-optimization of two ormore beam steering components (e.g., mirrors 1020, 1021) may beperformed. This specific implementation may comprise the first mirror,e.g., mirror 1020 making an incremental movement in a predefined rangein x direction, followed by the second mirror, e.g., mirror 1021,scanning the full area determined by a desired range in x direction anda desired range in y direction. When the area scan by the second mirroris complete, the first mirror makes another incremental movement in thesame direction and the second mirror performs another area scan. This isrepeated until the first mirror reaches the end of the desired range inx direction, when it incrementally moves in y direction and performsanother incremental scan in the x direction. The full mirror scanningprocess is complete when the first mirror has scanned the full areadetermined by the predefined ranges in x and y directions. As such, apower map and/or a calculated PSD map will be generated by the secondmirror for each position of the first mirror and, as such, optimalmirror positions determined in this way should be more accurate.

Alternatively, or in addition to controlling (e.g., scanning) one ormore beam steering components, further optimization of the output modepurity of a broadband radiation source can be achieved by controlling(e.g., incrementally scanning) the position of the gas cell 1040. Thegas cell movement may be enabled by a stage module and may compriselateral and/or angular movement in one or more directions. Furthermore,in an embodiment, the focusing lens 1030 may be mounted on a piezo stageor a stage module which allows the lens to move according to one or moredegrees of freedom. Such lens movement can further optimize the modepurity of the broadband output 1080.

In a further embodiment, an assembly comprising an optical element forreceiving and modifying radiation, a receiving element for receiving themodified radiation, and a gas environment enclosing the receivingelement is provided, wherein the assembly further comprises acontrolling element configured to stabilize a matching condition betweenthe optical element and the receiving element by adjusting either themodifying of the received radiation or adjusting a distance between theoptical element and the receiving element in dependency to a property ofthe gas environment.

In an example the optical element may be the focusing lens 1030 and thereceiving element a non-linear optical element such as a non-linearcrystal or the hollow core fiber HC-PCF 1041. The radiation may begenerated by a (monochromatic) pump laser, focused (modified) by theoptical element 1030 and received by the receiving element 1041 enclosedwithin the gas cell 1040. The gas cell 1040 may be configured to allowadjustment of one or more certain properties of the gas environment,such as pressure, temperature and/or gas composition. Typicallyadjustment of the gas environment is targeted to obtain a desiredresponse of the non-linear optical process of generating broadbandradiation, for example to adjust the wavelength spectrum of thebroadband radiation. However changes in the one or more properties(typically temperature and/or pressure of the gas environment) may havea direct impact on a matching condition between the optical element 1030and the receiving element 1041, such as a focus matching conditionassociated with an efficiency of coupling the radiation into the HC-PCF1041. In case the one or more properties of the gas environment arepressure and/or temperature variations, basic physical principles allowestimation of the corresponding variation of the focus matchingcondition. The refractive index ‘n’ of the gas depends on the pressure Pand temperature T of the gas according to equation 1 (EQ[1]):

$\begin{matrix}{{n^{2}\left( {\lambda,P,T} \right)} = {1 + {\frac{P}{P0}*\frac{T0}{T}*\left\lbrack {\frac{B1*\lambda^{2}}{\lambda^{2} - {C1}} + \frac{B2*\lambda^{2}}{\lambda^{2} - {C2}}} \right\rbrack_{{P0},{T0}}}}} & {{EQ}\lbrack 1\rbrack}\end{matrix}$wherein P₀ and T₀ are reference pressure and temperature values, C₁, C₂,B₁, B₂ are the Sellmeier coefficients of the gas and λ is the wavelengthof the pump laser radiation.

The variation in focal position of the pump laser radiation with respectto the entrance of the fiber 1041 due to the change in refractive indexcan be determined by basic optical analysis of the assembly (inparticular the power of the optical element and the distance between theoptical element and the receiving element). For example, a pressurechange from 15 to 17 bar of a xenon filling gas will cause a refractiveindex increase of approximately 0.0015 in case of a 1 μm wavelength pumplaser source. Using a simple optical model for a typical assembly designit can be determined that this causes a focus shift of 20-30 μm.

The pressure and/or temperature variation induced focus variation may bedetrimental for the coupling efficiency of the pump laser radiation intothe fiber 1041. This may translate into a reduced power of the broadbandradiation generated within the fiber 1041. It is hence desired toprovide focus controlling means, for example by incorporation of acontrolling element, such as an actuator, providing focus control basedon available information of one or more properties of the gasenvironment.

The optical element 1030 and/or gas cell 1040 may be configured to allowvariable focusing of the pump laser radiation with respect to theentrance of the receiving element HC-PCF 1041. For example, the focusinglens 1030 and/or gas cell 1040 may be movable by the controlling elementwithin a certain range along the optical axis of the receiving elementHC-PCF 1041 (longitudinal direction). Alternatively or additionally, thefocusing lens may comprise an optical surface (lens) having a variableoptical power (for example the lens/optical surface being deformable bythe controlling element) or a lens (elements) which may be movable bythe controlling element with respect to each other.

It is further proposed to periodically measure the pressure and/ortemperature of the gas by sensing means, determine a correspondingchange in refractive index of the gas and subsequently determine thevariation in focal position of the pump laser radiation with respect tothe entrance of the receiving element 1041 due to the change inrefractive index.

In an embodiment the assembly comprises a focus control system usingpressure and/or temperature measurement values as an input andoutputting a value corresponding to a control signal for one or morecontrolling elements (actuators) coupled to the focusing lens 1030and/or gas cell 1040. The control signal may be configured to provide afocal position change at least partially compensating the determinedvariation in focal position of the pump laser radiation with respect tothe entrance of the fiber 1041. The one or more actuators may move thefocusing lens 1030 and/or the gas cell 1040 along the optical axis. Theone or more actuators may position a lens (elements) comprised withinthe focusing lens 1030 such as to cause the focal position change. Theone or more actuators may deform an optical surface or lens (element)comprised within the focusing lens 1030 such as to cause the focalposition change. Alternatively or additionally, the one or moreactuators may control the position and/or optical power of an additionaloptical element (not shown) positioned in the radiation path upstream ofthe gas cell to cause the focal position change.

The focus control system may comprise functionality to determine achange in refractive index of the gas based on the pressure and/ortemperature measurement value, a reference value of the pressure and/ortemperature, the wavelength of the pump laser radiation, and/or thecomposition of the gas and/or the Sellmeier coefficients correspondingto the gas.

The focus control system may comprise functionality to determine thevariation of the focal position of the pump laser radiation with respectto the entrance of the fiber 1041 based on the determined variation ofthe refractive index of the gas and knowledge of one or more propertiesand/or positions of the optical elements used in coupling the pump laserradiation into the fiber 1041.

The focus control system as described herein allows automated focusadjustments in response to varying conditions of the gas (for example:temperature, pressure, gas composition) helping to assure efficientcoupling of the pump laser radiation into the fiber 1041. Hence thefocus control system adds to the stabilization of the power of thebroadband radiation being delivered by the fiber 1041.

In an embodiment, the power of the broadband radiation is periodicallymeasured at the output of the fiber 1041 and used instead or in additionto pressure and/or temperature measurement readout to provide thecontrol signal for the one or more actuators. In an embodiment, a ratiobetween the power at the output of the fiber and at the entrance of thefiber is determined. The determined ratio may be used to provide thecontrol signal for the one or more actuators.

FIG. 11 illustrates an embodiment of a detection unit that comprises aspatial filter 1152, which may comprise (for example) a pinhole or asingle mode fiber, and a power measuring device 1153. Similar to theembodiment described in relation to FIG. 10 , a reference beam 1181 isdirected by the beam splitter 1151 from the broadband output beam. Thespatial filter 1152 is configured to remove HOM content of the broadbandoutput such that only the fundamental transverse mode is measured andmonitored. Since the fundamental transverse mode and HOMs have differentdivergence angles and mode field widths (e.g., diameters), only thefundamental mode of the output beam can be efficiently coupled into thesingle mode fiber and the HOMs are either not coupled into thesingle-mode fiber or not guided to the power meter for powermeasurement. In a similar manner, a pinhole with a carefully chosensize, only allows the fundamental transverse mode to be transmitted andthus effectively removes the HOMs of the output beam.

After being spatially filtered, the power in the fundamental transversemode LP₀₁ is measured by the power measuring device 1153 placed behindthe spatial filter in the detection unit. One or more additionalbandpass filters (not shown) or other filter arrangement may be used toselect one or more desired spectral ranges for power measurement. Whenone or more pump coupling conditions are suboptimal, the output power inthe fundamental transverse mode begins to drop. As soon as the power inthe fundamental mode falls below a predefined power threshold, a controlsignal is generated and/or varied and sent to the control unit 1170. Thecontrol unit 1170 will activate an optimization routine (e.g., asdescribed above) to optimize one or more pump coupling conditions suchthat the output power in the fundamental transverse mode increasessufficiently (above the threshold) indicative of improved output modepurity.

It should be noted that a power drop may be caused partly by thermaland/or vibrational drifts of the collimating lens 1131 and/or one ormore other downstream optical components such as the optical beamsplitter 1151 illustrated in FIG. 11 . Therefore, one or more beamalignment measuring devices (not shown) can be provided in the detectionunit 1150 so as to either continuously or intermittently monitor theposition of the collimated output beam 1180 and the reference beam 1181.When the position of collimated output beam 1180 and/or the referencebeam 1181 are confirmed to have drifted, the position of the spatialfilter 1152 can be optimized accordingly to compensate for the drift.

According to an embodiment, as illustrated in FIG. 12 , the detectionunit 1250 comprises a beam shape measuring device 1253 (more generally abeam shape and/or size measuring device) which measures various (e.g.,far-field) shape/size parameters of the incident reference beam 1281,such as one or more selected from: width/diameter/radius, ellipticity,centroid position, etc. The beam shape measuring device 1253 may be ascanning-slit beam profiler or a CCD camera, for example. One or moreadditional bandpass filters may be used to select one or more desiredspectral ranges for beam profile measurement. Since the fundamentaltransverse mode of a HC-PCF has a Gaussian or near Gaussian fielddistribution and HOMs have non-Gaussian field distributions, one or morebeam shape parameters such as ellipticity and/or beam width (e.g.,diameter) can be used (separately or in combination) to evaluate thefundamental mode purity. Once measured, the one or more beam shapeparameters are sent to the processing unit 1260 for data processing andevaluation. For example, if it is evaluated that the measured beamellipticity is greater than a predefined ellipticity threshold, the modepurity of the broadband output beam 1280 is confirmed to be sub-optimal.The ellipticity threshold may be set to a value in a range between 1.04and 1.20, for example. Alternatively, or in addition, the evaluation maycomprise comparing the measured beam width (e.g., diameter) with areference value of a collimated Gaussian beam calculated using therelevant parameters of the HC-PCF 1241 and the collimating lens 1231. Ifthe difference between the measured beam width and the reference beamwidth is greater than a certain threshold, the mode purity of thebroadband output beam 1280 is confirmed to be sub-optimal. In anembodiment, both size and ellipticity are measured and evaluated againstrespective thresholds as one of these parameters alone may not always becompletely indicative of mode purity. Alternatively, or in addition,such a method may monitor one or more Laguerre-Gaussian mode shapes ofthe beam and fit these to Laguerre-Gaussian polynomials indicative ofmode purity (or otherwise). Alternatively or in addition, one or moreZernike polynomial shapes can be monitored and fitted in a similarmanner. Once sub-optimal mode purity is confirmed, a control signal isgenerated and/or varied by the processing unit 1260 and sent to thecontrol unit 1270 for pump coupling optimization routine.

In an embodiment, an optical lens 1252 may be comprised within thedetection unit to image the end facet of the HC-PCF 1241 onto the beamshape measuring device 1253. In comparison to the example describedabove, where far-field distribution of the output mode is evaluated,this example instead uses near-field distribution of the HC-PCF outputfor mode evaluation. Similarly, the ellipticity and/or the width (e.g.,diameter) of the near field distribution are evaluated against one ormore theoretical and/or empirical values. One or more empirical valuesderived experimentally might be more reliable in some cases.

According to an embodiment, as illustrated in FIG. 13 , the detectionunit 1350 comprises a spectrum measuring arrangement; specifically: amultimode fiber 1352, and an optical spectrum measuring device (e.g.,optical spectrometer or optical spectrum analyzer) 1353. One end of themultimode fiber 1352 is placed into the beam path and used to receive atleast part of the reference beam 1381. The other end of the multimodefiber is optically connected to the optical spectrum measuring device1353 which is configured to analyze spectral characteristics of thereference beam 1381. The intensity of the reference beam 1381 may beattenuated/controlled by a neutral density (ND) filter (not shown) toavoid damage of the fiber facet and/or saturation of the opticalspectrum measuring device. In a different embodiment, the multimodefiber 1352 may not be required. The reference beam may be free-spacecoupled into the optical spectrum measuring device.

Similar to the embodiment where measured spectral parameter values(e.g., PSD values) are compared with reference values in correspondingspectral ranges, in this embodiment, the spectral parameter values(e.g., measured spectrum) may be compared with a reference spectrumwhich may be acquired when the mode purity of the broadband output beamis known to be optimal. Depending on the extent of difference betweenthe measured spectrum and the reference spectrum, a control signal willbe generated and a pump coupling optimization performed.

In an embodiment, as illustrated in FIG. 14 , radiation 1481 leakingthrough the fiber cladding is collected, e.g., by a multimode fiber1452. This radiation 1481 may be collected from only a section of HC-PCF1441, e.g., at or near an end section of HC-PCF 1441 (e.g., at or nearthe output end), where the outer coating may be stripped. The collectedleaking radiation 1481 is then guided to an optical spectrum measuringdevice 1453 for spectrum measurement. Higher order fiber core modes inan HC-PCF will experience a higher confinement loss than the fundamentalLP₀₁ core mode as they propagate along the fiber. Hence, in the casewhere the mode purity has degraded or is sub-optimal, more power willleak through the cladding structure, giving rise to an increasedamplitude of the measured spectrum. Therefore, the amplitude of themeasured spectrum can be used to assess whether the mode purity of thebroadband output is optimal or not (e.g., by comparison to a threshold).If the mode purity turns out to be sub-optimal, a control signal will begenerated by the processing unit and a pump coupling optimizationroutine will be activated by the control unit.

In addition to embodiments based on improving coupling based on a poweror energy metric or beam shape metric, other parameters of the emittedradiation may be measured such as polarization extinction and/orpolarization angle. Note that these latter parameters cannot be measureddirectly from a beam nor radiation emitted from the cladding of thefiber, and are only partially measurable from the beam or the radiationcoming from the secondary axis of a beam splitter.

The above pump coupling optimization methods relate predominately to thefine alignment aspects of the alignment of the pump laser beam withrespect to the HC-PCF. Improvements predominately for coarse alignmentwill now be described. It is to be noted that the solution spaces of theabove pump coupling optimization method and the hereinafter discussedimprovements may overlap. For example, the above discussed pump couplingoptimization method may also have its benefits for coarse alignment, andthe hereinafter discussed improvements may also provide some finealignment. The proposed methods and apparatuses use one or more suitabledetectors (e.g., photo-diodes and/or optical power meters) to monitorthe HC-PCF cladding, and more specifically, the radiation escaping theHC-PCF radially. The concepts in this embodiment are similar to that forthe fine alignment (mode purity evaluation) described in relation toFIG. 14 . It can be appreciated that the methods of any of the foregoingembodiments may be used independently of or in combination with any ofthe embodiments disclosed above, in the latter case such that any of theforegoing embodiment is used for an initial coarse alignment and any ofthe preceding embodiments are used subsequently (i.e., when coarselyaligned) for fine alignment. Please note that the discussed coarsealignment may also be used for schemes in which a beam of radiation mustbe aligned with respect to a solid core of a photonic crystal fiber thathas a cladding region around core of a solid material.

FIG. 15A schematically depicts a coarse alignment arrangement CAaccording to such an embodiment. The coarse alignment arrangement CAcomprises an optical element, in this example a positive lens POL, tofocus a radiation beam LB onto an input surface INS of an HC-PCF. As inthe examples of FIG. 7 , the HC-PCF has a (hollow) fiber core FCO and aninner cladding waveguide structure (fiber cladding CLA) surrounding thefiber core FCO. The input surface INS delimits one end of the HC-PCF andis configured to receive the radiation beam LB in order to couple atleast a part of the radiation beam LB into the fiber core FCO.

It is to be noted that the optical element may be any type of opticalelement and is not necessarily limited to a positive lens POL. Forexample, the optical element may be an off-axis parabolic mirror. Thecoarse alignment arrangement CA further comprises a detector, such as aphotosensor PHS, arranged near or on the fiber cladding CLA of theHC-PCF. The photosensor PHS is here embodied as a photodiode, but can beany other type of sensor of radiation or other electromagnetic energy.The photosensor PHS is arranged such that it can receive radiation fromthe radiation beam LB that is coupled into the fiber cladding CLA at theinput surface INS. The photosensor PHS is further configured to output asignal SI that is representative for the amount of radiation received bythe photosensor, so that the output signal SI is representative for theamount of radiation coupled into the fiber cladding CLA. Hence, thephotosensor measures the radiation scattering out of the fiber claddingCLA that is present due to misalignment of the radiation beam LB and thefiber core FCO. Optionally, there may be an optical filter before thephotosensor PHS, to filter unwanted wavelengths/polarizations and/orreduce amount of radiation (e.g., to within the dynamic range of diode).

FIG. 15B depicts in more detail the coarse alignment arrangement CA ofFIG. 15A by omitting the optical element POL and focusing on the freeend of the HC-PCF, where the input surface INS receives the radiationbeam LB. In FIG. 15B, it can be clearly seen that the radiation beam LBis not completely aligned with the fiber core FCO and thus a portion ofthe radiation beam LB is coupled into the fiber core FCO and anotherportion of the radiation beam LB is coupled into the fiber cladding CLA.

In some systems, measuring the misalignment between radiation beam LBand fiber core FCO can be measured by fiber tapping in which the amountof radiation coupled into the fiber core FCO is measured by altering,e.g. damaging, the HC-PCF to tap a portion of the radiation trappedinside the fiber core FCO and direct this portion to a detector orsensor. However, this leads to transmission loss and may cause aspectral change and/or a change of the polarization extinction ratio.

In this embodiment, it is proposed to measure the radiation coupled intothe fiber cladding CLA rather than the radiation coupled into the fibercore. On the right of FIG. 15B, the input surface INS is shown with theradiation beam LB on the left of the HC-PCF. Below the HC-PCF a diagramis shown depicting the output signal In when the radiation beam LB ismoved in the X-direction from the left side of the HC-PCF to the rightside of the HC-PCF, but it is noted here that similar diagrams can beobtained by moving in other degrees of freedom. The radiation beam LB isshown to begin incident mostly outside of the fiber cladding CLA,corresponding to alignment position AL₁ (a specific illustration of therespective arrangement of radiation beam LB, fiber core FCO and fibercladding CLA is shown directly below each value). At alignment positionAl₂, the radiation beam LB is mostly incident on the fiber cladding CLAresulting in an increase in the value of the output signal In.Subsequently the radiation beam LB will be incident to the fiber coreFCO resulting in a decrease in the value of the output signal In; thiscorresponds to the best alignment position ALB. Finally, the radiationbeam LB will be incident on the fiber cladding CLA again resulting in anincrease in the value of the output signal In at alignment position AL₃.Hence, the best alignment between radiation beam LB and fiber core FCOis obtained when the output signal In is at a minimum value In_(min) inbetween a first maximum value In_(max1) and the second maximum valueIn_(max2) when the radiation beam LB is maximally coupled into the fibercladding CLA.

It is noted here that the minimum value In_(min) is not necessarily azero value. A non-zero value for the minimum value In_(min) may verywell be possible or even likely, particularly for a coarse alignmentstage, e.g., as in practice there may always be a certain level ofradiation scattering from the core. The signal In_(min) may also be usedduring operation of the broadband radiation source to monitor a healthof the system and/or of the alignment and/or of the fiber.

It should also be appreciated that any scan will in fact be a 2dimensional scan over the input surface INS. As such, the model will bea three dimensional model as illustrated in FIG. 15C, which shows athree dimensional model similar to the 2 dimensional plot of FIG. 15B,and three different offset cross-sections thereof (here the maxima areshown as equal in all directions, although this may not be the case, asshown in FIG. 15B). The scanning algorithm will therefore aim to findthe position corresponding to a common minimum between maxima in alldirections on the X/Y plane.

The coarse alignment strategy may operate in a feedback loop based onthe measured output signal and a control signal for a controller whichcontrols the position of the beam with respect to the input facet of theHC-PCF, so as to find this minimum value In_(min). Such a method may usea search algorithm which searches automatically whether the input beamis aligned sufficiently with respect to the input of the hollow HC-PCF.Such a method may comprise a spiral scan to find a region bounded bymaximum values (e.g., forming an annular region), and finding theminimum within this region.

FIG. 16 schematically depicts a further coarse alignment arrangement CA.FIG. 16 shows respectively, on the left, a side view and, on the right,a front view of an HC-PCF and a photosensor PHS (or other detector)arranged around the HC-PCF. The HC-PCF comprises a hollow fiber core anda fiber cladding surrounding the fiber core as depicted in otherfigures, but not shown here explicitly. The HC-PCF further comprises aninput surface INS configured at one end of the HC-PCF to receive aradiation beam in order to couple at least a part of the radiation beaminto the fiber core. The photosensor PHS is arranged to receiveradiation from the radiation beam being coupled into the fiber claddingat the input surface INS, wherein the photosensor is configured tooutput a signal that is representative for the amount of radiationreceived by the photosensor.

The photosensor PHS may have multiple separate areas that are capable ofdetecting the amount of radiation that falls on the individual multipleseparate areas. If such a photosensor PHS with multiple separate areasis arranged around the HC-PCF information may be obtained with respectto the direction in which a specific amount of radiation is outputtedfrom the fiber cladding. This directional information may be used tosteer an alignment of the radiation beam into a direction that dependson the detected directional information.

In the embodiment of FIG. 16 , the photosensor PHS is arranged on thefiber cladding near the input surface INS. The photosensor PHS extendsin radial direction RAD of the HC-PCF about a longitudinal axis LAF ofthe HC-PCF over and angle α, which in this case is larger than 180degrees, even larger than 270 degrees, and almost is 360 degrees. Suchan embodiment may advantageously be used to increase the signal-to-noiseratio as for an increased angle α more radiation is received by thephotosensor PHS.

FIG. 17 schematically depicts a further coarse alignment arrangement CA.The coarse alignment arrangement of FIG. 17 is similar to that of FIG.16 and only the front view is depicted here to explain the maindifference between the two embodiments. The main difference is that inthe embodiment of FIG. 16 , the coarse alignment arrangement CAcomprises two photosensors, namely a first photosensor PHS1 and a secondphotosensor PHS2, each having the same function. The first and secondphotosensor PHS1, PHS2 are provided distributed substantially evenly asseen in a radial direction RAD along the periphery (e.g., circumference)of the HC-PCF, wherein each photosensor extends in the radial directionRAD of the HC-PCF about a longitudinal axis LAF of the HC-PCF over anangle α and β, respectively, which angles are smaller than 180 degrees,but desirably larger than 90 degrees.

The first photosensor PHS1 provides a first output signal In1 and thesecond photosensor PHS2 provides a second output signal In2. Acombination of the signals In1 and In2 can be used in a similar manneras the output signal In of the photosensor PHS of the embodiment ofFIGS. 15 and 16 . However, an advantage of this arrangement is that acontrol unit receiving both signals In1 and In2 can also determine alinear, or weighted, difference of the signals In1 and In2, which can beused to determine in which direction the radiation beam needs to bedisplaced or tilted to align the radiation beam with the fiber core.

Although not shown, it can be envisaged that in an embodiment, three ormore photosensors may be provided and distributed substantially radiallyevenly along the periphery of the optical fiber, wherein eachphotosensor extends in radial direction of the optical fiber about alongitudinal axis of the optical fiber over an angle smaller than 360/ndegrees, where n is the amount of photosensors, for example an anglesmaller than 120 degrees in case of three photosensors.

In a variation of the arrangement depicted in FIG. 17 , the secondphotosensor PHS2 may be replaced by a mirror element used in combinationwith only a single photosensor PHS1. The mirror element reflectsradiation coupled into the fiber cladding at the input surface towardsthe photosensor PHS1. As a result thereof the signal-to-noise ratio maybe improved. The mirror element is not necessarily arranged on thefiber, but may be arranged around the fiber at a distance thereofinstead. The mirror element may also be accompanied by one or more othermirror elements all configured to reflect radiation coupled into thefiber cladding towards the photosensor PHS1.

An advantage of this coarse alignment arrangement is that radiation canbe coupled into the core of the optical fiber after misalignment, e.g.due to replacing of components or drift, which coupling can be doneinline without having to change or disconnect the system.

FIG. 18 schematically depicts an optical system OS with a specific beamsteering arrangement or optical manipulation unit OMU usable for actualcontrol of the beam on the input face in a coarse alignment arrangementand/or in any of the fine alignment embodiments as disclosed herein.

The optical system OS comprises a radiation source LIS and an opticalmanipulation unit OMU. The radiation source LIS provides a radiationbeam LB to the optical manipulation unit OMU using a fiber FI and anoutput connector OC, which output connector OC may comprise a collimatorto provide a collimated radiation beam LB to the optical manipulationunit OMU. The radiation source LIS may be a white light source or asupercontinuum source.

The optical manipulation unit OMU comprises an input device IDconfigured to receive the output connector OC. It will be appreciatedthat the input device ID and the output connector OC have been depictedhighly schematic, but that both components may comprise featuresallowing the output connector OC to be releasably, but rigidly connectedto the input device ID, allowing to replace the output connector OC orto disconnect the output connector OC and subsequently connect theoutput connector OC again.

The optical manipulation unit OMU further comprises one or more opticalelements configured to manipulate the radiation beam LB. Depicted as anexample of such optical elements in FIG. 18 are a mirror MI directingthe radiation beam to a filter unit FU configured to filter theradiation beam passing the filter unit FU. Filtering may comprisespectral filtering, polarizing filtering and/or overall attenuation ofthe radiation beam.

It is mentioned here that the presence of a filter unit does not meanthat other optical elements may not have a filtering function as well,e.g. in the form of a reflective or transmissive band pass filter.Hence, the mirror MI may have such alternative or additional filteringfunction.

In this embodiment, downstream of the filter unit FU a radiation beamtilt adjuster TA is provided to adjust a propagation direction of theradiation beam LB. The radiation beam tilt adjuster TA comprises a firstwedge prism WP1 and a second wedge prism WP2 arranged in series, whereineach wedge prism WP1, WP2 comprises a respective tilt actuator A1, A2 torotate the corresponding wedge prism WP1, WP2 about its respectiveoptical axis, which in FIG. 18 extends mainly in the X-direction. Thetilt actuators A1, A2 are part of a tilt actuation system.

In this embodiment, arranged downstream of the radiation beam tiltadjuster TA, a radiation beam displacement device DD is provided todisplace the radiation beam LB. The radiation beam displacement deviceDD comprises a plane-parallel plate PP and a displacement actuationsystem to rotate the plane-parallel plate PP about a first axis usingactuator A3 and a second axis using actuator A4, which first and secondaxes are substantially perpendicular to each other and to thepropagation direction of the radiation beam. The first axis may forinstance be substantially parallel to the Y-direction and the secondaxis may for instance be substantially parallel to the Z-direction asthe propagation direction of the radiation beam LB is substantially inthe X-direction.

The optical manipulation unit OMU further comprises a control unit CUconnected to the tilt actuation system (A1, A2) and the displacementactuation system (A3, A4) in order to adjust the propagation directionof the radiation beam and to displace the radiation beam in order todirect the radiation beam LB towards an input connector INC. The inputconnector INC is received in an output device OD of the opticalmanipulation unit OMU and may comprise a coupling device to coupleradiation into fiber HC-PCF. Similarly to the input device ID and theoutput connector OC, the output device OD and the input connector INCare depicted highly schematically here and thus may comprise featuresallowing them to rigidly, and possibly releasably, connect to eachother, allowing the input connector INC to be replaced by a new ordifferent input connector INC or to be disconnected and connected again,e.g. for maintenance.

The fiber HC-PCF may, as in this example, comprise a first fiber portionand a second fiber portion connected to each other using a connectorCON. The connector CON or the fiber HC-PCF may be configured to direct aportion, desirably a small portion of the radiation, passing theconnector CON to a detector DE to determine a radiation intensity of theradiation beam in the fiber HC-PCF which radiation intensity is ameasure for the amount of radiation that is coupled into the fiber bythe coupling device of the input connector INC. Hence, the determinedradiation intensity of the radiation beam in the fiber HC-PCF can beused to operate the control unit CU to control the tilt actuation systemuntil the radiation beam is received by the input connector INC.

In an embodiment, the control unit CU may be configured to control thetilt actuation system such that the radiation beam is moved along aspiral pattern, in this case in the Z-Y plane, in order to find a firstestimate of a desired propagation direction of the radiation beam to bereceived by the input connector INC and subsequently the tilt actuationsystem and/or the displacement actuation system is controlled to movethe manipulated radiation beam around the first estimate in order tofind an improved first estimate of the desired propagation directionand/or displacement of the radiation beam. Moving the radiation beam ina spiral pattern may be accomplished by rotating the two wedge prismsWP1, WP2 at different angular velocities. The first and second estimatesmay both be coarse alignment estimates, with the fine alignmentstrategies otherwise disclosed herein applied for fine alignment.Alternatively, the first estimate may relate to the coarse alignment andthe second estimate may relate to the final alignment. In the latterexample, the fine alignment may rely on one of the other measurementstrategies described herein.

In accordance with an embodiment, there is provided a timing controlsystem configured for controlling timing of pump laser pulses and/orbroadband output pulses of a HC-PCF based broadband radiation source.Timing control of laser pulses is often desirable in applications whereprecise temporal location of laser pulses with respect to a timingreference is desired and is typically achieved using for examplemicro-processor or micro-controller based technologies. A typical timingcontrol system may comprise one or more micro-processors, a centralprocessing unit (CPU), and a memory unit. When such a micro-processorbased timing control system is used for controlling timing of a HC-PCFbased broadband radiation source, timing of the pump laser pulse, thebroadband output pulse and the generated signals from one or moreoptical clients, e.g., optical sensors, can be determined and/orsynchronized. However, using prior timing control systems to controltiming of HC-PCF based broadband radiation sources may have manytechnical challenges or drawbacks. First, a HC-PCF based radiationsource comprises multiple components which are often located far apartfrom each other. For example, the pump laser, typically comprising aseed laser, a pre-amplifier, a pulse stretcher, a power booster and apulse compressor, is connected to a supercontinuum fiber which could belocated more than 10 meters away. Communications between suchcomponents, e.g., between a seed laser and a supercontinuum fiber, cancause non-negligible time delay. Further, the timing of the pump laseris complex due to its complex optical architecture (e.g., an opticalsystem comprising a seed laser, a pre-amplifier, a pulse stretcher, apower booster and a pulse compressor). Consequently, each component ofthe pump laser will have an impact on the timing of the pump laserpulse. The timing of the broadband output pulse is also complex due tothe intra-pulse group delay dispersion resulting in differentwavelengths having different timings. Those complex timings imposestringent requirements on the performance (e.g., timing accuracy) of thetiming control systems and are typically fulfilled with a complexcontrol architecture. Furthermore, the generated signal from each of theoptical clients, e.g., optical sensors, needs to be processed beforebeing usable by other control units for other components. To fulfil thisrequirement, prior timing control systems are often equipped with signalprocessing functionality, thereby resulting in an even more complexcontrol architecture.

The above-mentioned drawbacks may result in difficulties in accurateprediction and/or modelling of the time delay between any twocomponents, e.g., the seed laser and one of the optical clients formonitoring the supercontinuum output. Although the time delay determinedby the timing control system could be calibrated against measurementdata such that timing error can be corrected, this approach, however, isnot practical for HC-PCF based radiation sources as their laser pulsesare too short to be accurately measured. To obtain the best possibletiming performance, extensive control firmware and/or softwareconfigured to operate within predefined margins is typically required.The extensive control firmware and/or software together with the complexmicro-processor based hardware makes the whole timing control systemoverly complex and expensive. Since a firmware or software based controlsystem is more prone to errors, it takes a large time and effort to makeit robust.

In accordance with an aspect of the present disclosure, embodimentsdescribed below provide better solutions to one or more of theaforementioned, or other, problems. A significant advantage of one ormore of the following embodiments over prior timing control systems isthat the use of micro-processors or similar technologies can beprevented.

FIG. 19 schematically illustrates a timing control system configured fortiming control of a HC-PCF based broadband radiation source, inaccordance with an embodiment. With reference to FIG. 19 , the timingcontrol system may comprise a pressure sensor 1944 (e.g., a very highspeed pressure sensor) configured to detect or monitor pressure changeof the gas cell 1940. In this embodiment, the pressure sensor 1944 maybe held in the vicinity of the output end of the HC-PCF 1941 by asupporting structure 1945 which is connected to the inner side of thegas cell wall 1946. In some embodiments, the pressure sensor 1944 may bemounted directly on the inner side of the gas cell wall 1946. The gassensor 1944 may communicate with one or more external devices via one ormore signal cables which either pass through the gas cell wall 1946 in asealed manner or connect to one or more external cables via one or morefeedthrough connectors.

With continued reference to FIG. 19 , the pump laser beam 1911comprising a train of pump laser pulses 1912 is focused by a focusinglens 1930. The focused pump laser beam 1911 passes through an inputwindow 1942 of the gas cell 1940 before coupling into the core of theHC-PCF 1941. While propagating along the fiber, each pump laser pulse1912 is spectrally broadened to a broadband output pulse 1982 via one ormore aforementioned nonlinear optical processes. After leaving the gascell 1940, the broadband output beam 1980 comprising a train ofbroadband output pulses 1982 is collimated by a collimation lens 1931.From the onset of the one or more nonlinear processes, the spectralbandwidth of the pump laser pulse 1912 continues to increase (e.g.,spectral broadening) until the spectrally broadened pulse exits thefiber. Once leaving the fiber, the broadband output pulse 1982 maytravel a short distance inside the gas cell 1940 before exiting theoutput window 1943 of the gas cell 1940. When entering into the gas cell1940, the broadband output pulse 1982 generates a pressure wave insidethe gas cell 1940. The amplitude of such a pressure wave can be affectedby many factors, such as the operating conditions of the gas cell 1940(e.g., gas cell pressure, gas type, etc.) and laser parameters of thebroadband output pulse 1982 (e.g., pulse energy, pulse spectrum, pulsewidth, etc.). The pressure wave (temporally) alters the internalpressure distribution of the gas cell 1940. The resultant pressurechange at the location of the pressure sensor 1944 is detected by thepressure sensor and is subsequently converted by the pressure sensor1944 to an electrical signal. In some embodiments, the electrical signalmay be used as an output pulse trigger signal which indicates the timingof a broadband output pulse 1982 being generated. Additionally oralternatively, such an electrical signal may be sent to the processingunit 1060, 1160, 1260, 1360, 1460 and/or control unit 1070, 1170, 1270,1370, 1470 of the broadband radiation source such that differentfunctionalities can be realized, for example, outputting a burst ofbroadband pulses.

Note that a pressure wave is also generated when the pump laser pulse isadmitted into the gas cell 1940 before coupling into the HC-PCF 1941.Therefore, in some embodiments, timing of the pump laser pulse 1911entering into the gas cell 1940 may also be determined using the samepressure sensor 1944 and/or an additional pressure sensor (not shown)e.g., located in the vicinity of input window 1942.

In conjunction with the timing of the broadband output pulse 1982, arelative time delay between the pump laser pulse 1911 and the broadbandoutput pulse 1982 may be determined. Note that since the broadbandoutput pulse 1982 is intrinsically synchronous (in a fixed timerelation) with the pump laser pulse 1911, the electrical signal of thebroadband output pulse 1982 is therefore also synchronous with theelectrical signal of the pump laser pulse 1911. The two pulse trains,i.e. the pump laser pulse train and the broadband output pulse train,are offset in time by an amount of the aforementioned time delay. Insome embodiments, the timing control system may further comprise anadjustable optical delay line configured to adjust or minimize the timedelay between the two pulse trains.

It can be appreciated that the methods described above in relation toFIG. 19 may be used independently of or in combination with any of theother embodiments disclosed herein. Where used in combination, the modepurity based embodiments may be used for an initial alignment (coarseand/or fine) and any of the preceding embodiments used subsequently(i.e., when the radiation source is properly aligned) for timingcontrol.

For some metrology or inspection tools such as the aforementionedscatterometry based metrology tools, the performance of a metrology toolcan be strongly affected by one or more polarization properties of theillumination radiation of the tool. Such one or polarization propertiesmay include polarization extinction ratio (PER) or polarization quality,polarization stability, orientation of predominantly linearly polarizedradiation, etc. The PER is defined as the power ratio of twoperpendicular polarizations which are often called transverse electric(TE) and transverse magnetic (TM) and is typically used to characterizehow good a linear polarization is. The polarization stability is used tocharacterize how stable a polarization state can be maintained overtime. Due to component aging and/or movement, the polarization of theillumination radiation would change over time, resulting in rotation ofpolarization and/or PER degradation. If an illumination beam with a poorPER is used in a polarization sensitive metrology tool, e.g., ascatterometer, the optical power in the unwanted polarization directionwill not contribute to the measurement and can even cause backgroundscattering, thereby reducing the detection signal-to-noise ratio (SNR).Moreover, since only the optical power in the desired polarizationdirection is used for measurement, the power efficiency of the metrologytool is low. Similarly, if an illumination beam with an unstablepolarization is used in e.g., a scatterometer, variations in thereceived polarization typically result in power fluctuations atsubstrate level, impairing the fidelity of the metrology tool.Therefore, it is desirable to use an illumination source which is ableto provide a good polarization stability.

When HC-PCF based broadband radiation sources such as those described inFIGS. 10 to 14 are used as the illumination source in e.g., ascatterometer, a linearly polarized broadband output beam with a goodPER is desirable. Since the broadband output beam inherits polarizationproperties predominantly from the pump laser beam, a linearly polarizedpump laser beam with a good PER may be used. For a perfectly made andstraight HC-PCF, mounted without stress, the PER of the broadband outputbeam is expected not to depend on the input (pump) polarizationdirection. However, when coupling a low pump power through a HC-PCF(i.e. no spectral broadening occurs in the HC-PCF), the PER of thetransmitted pump laser beam can periodically vary with the input (pump)polarization direction. This is explained by the fact that slightasymmetries in the HC-PCF will result in a small optical birefringence,thereby making the fiber effectively a long polarization retarder (orwave-plate) with a fast axis and a slow axis. The slight asymmetries inthe HC-PCF could be due to fabrication tolerances and/or mountinginduced fiber stress. Fabrication tolerances can result in fluctuationsin the structural core width (e.g., diameter) of HC-PCF, therebyeffectively making the core slightly, e.g., elliptical. Consequently,orthogonal input polarizations may see different modal indices (i.e.forming a fast and a slow axis) and experience different attenuation.When the polarization direction of the pump laser beam matches witheither the fast or slow axis of the fiber, the polarization fidelity ofthe pump beam is maintained. However, where there exists a mismatchbetween the polarization direction of the pump laser beam and the fastor slow axis of the fiber, the PER of the transmitted pump laser beamwill be degraded.

Therefore, to obtain a linearly polarized broadband output beam with ahigh PER, it is desirable to accurately align the polarization directionof the pump laser beam with a desired birefringence axis of the fiber.For an industrialized laser product, the polarization direction of thepump laser beam is fully optimized during the factory build. Thecontrol/optimization of the polarization direction of the pump laserbeam is typically achieved at low power levels and using a polarizationcontrolling device, e.g., a half-wave plate (HWP), which can be locatedin the beam path of the pump laser beam before the in-coupling in theHC-PCF. While the polarization direction of the pump laser beam isrotated by the HWP, the PER of the pump laser beam after propagatingthrough the fiber is measured with a polarimeter. However, whenever akey component (e.g., gas cell or HC-PCF) is replaced e.g., for thepurpose of maintenance or repair, the polarization direction of the pumplaser beam with respect to the desired fiber axis needs to bere-optimized. This means a polarimeter will be used to characterize andconfirm the laser PER during and after re-optimization. In addition,custom-designed tooling may also be used to access and/or controlcertain component(s), e.g., the HWP in the packaged laser product.Although it could be possible to pre-characterize the axis orientationof the HC-PCF and use such information to align the absolute rotation ofthe fiber, any change of the pump polarization can still lead todegradation of output PER. Hence, a polarimeter will still be used,e.g., as a permanent in-product diagnostic, to monitor the stability ofthe pump polarization which is manifested by the PER of the transmittedpump laser beam. A polarimeter is an expensive and bulky diagnosticdevice. To equip each HC-PCF based broadband laser source with apolarimeter increases the cost and imposes limitation on the footprintof the laser product.

In accordance with an aspect of the present disclosure, a method isproposed to solve one or more of the aforementioned, or other, problems.The proposed method is based on the finding that, when a linearlypolarized beam of radiation propagates through a waveguide structurehaving an asymmetric modal index profile, e.g., for example a fiberhaving a (slightly) elliptical cross-section, the PER of the output beamand the output power are strongly correlated. Therefore, it is possibleto use the power measured at the fiber output to indirectly evaluate orinfer how well the polarization of the pump laser beam is aligned with adesired axis of the fiber. If the relation between the angular offsetbetween the polarization direction of the pump beam and the desiredfiber axis, and the power at the fiber output is known, it is alsopossible to use the power measured at the fiber output to determine theangular offset between the polarization direction of the pump beam andthe desired fiber axis at the fiber input. Since power measuring devicesare the most common diagnostic equipment and much less expensive thanpolarimeters, the method is thus advantageous over the prior methods asit can obviate the need of a polarimeter.

The correlation of output power with PER may be explained by astructural feature of the HC-PCF, the modal index, and attenuation ofthe fundamental guided mode (LP₀₁) can be approximated by:

$n_{01} = {{\sqrt{1 - \left( {\frac{u_{01}}{\pi} \times \frac{\lambda}{D}} \right)^{2}}{and}\alpha_{01}} \sim \frac{\lambda^{3}}{D^{4}}}$where u₀₁ is the 1^(st) zero of the Bessel function of first kind J₀, λis the wavelength and D the inscribed core diameter. As such, modepurity may also be monitored in this manner.

FIG. 20 schematically illustrates a polarization control systemconfigured for polarization control of a HC-PCF based broadbandradiation source, in accordance with an embodiment. With reference toFIG. 20 , the polarization control system may comprise an HWP 2029 forcontrol of the polarization of the pump laser beam 2011. The HWP 2029may be placed anywhere between the pump laser 2010 and the input window2042 of the gas cell 2040. In the case where a pump focusing lens 2030is used to focus the pump laser beam 2011 into the core of the HC-PCF2041, it is desirable to place the HWP 2029 before the focusing lens2030 as this could avoid the HWP 2029 being potentially damaged by thefocused laser beam with a higher peak intensity.

In an embodiment, the polarization control system may further comprise abeam splitter 2051 placed on the beam path of the output beam 2080,desirably after the collimation lens 2031 to avoid potential damage. Thebeam splitter 2051 may split part of the output beam 2080 and mayreflect it into a detection unit 2050 for diagnostic purposes. The partof the output beam 2080 reflected off the beam splitter 2051 may belabelled as a reference beam 2081. The detection unit 2050 may comprisea power measuring device 2053 such as a power meter that measures thepower of the reference beam 2081. Before being received by the powermeasuring device 2053, the reference beam 2081 may pass through one ormore optical filters 2052 that are used to select a desired range ofwavelengths from the spectrum of the reference beam 2081. Note that thebeam splitter 2051 impacts negligibly on the optical characteristics ofthe reference beam 2081 other than imposing a power splitting ratio tothe output beam 2080.

In some embodiments, the polarization control system may be operated ina high power or energy regime where the pump laser power/energy is highand the pump pulse will experience significant spectral broadening aftertraversing the fiber. In some other embodiments, the polarizationcontrol system may be operated in a low power/energy regime where thepump laser power/energy is low and the pump pulse will not undergospectral broadening while traversing the fiber. When operated in the lowpower regime, the output beam 2080 has substantially the same opticalspectrum as that of the pump laser beam. Since the pump laser beamtypically has a narrowband spectrum, spectral filtering may not benecessary and thus one or more optical filters 2052 may not be used. Incontrast, when operated in the high power regime, the output beam 2080has a broadband optical spectrum, significantly broader than that of thepump laser beam. In this case, spectral filtering may be required toonly measure the power in a desired spectral range and therefore one ormore optical filters 2052 may be used. In some embodiments, the beamsplitter 2051 may be removed and the detection unit 2050 may be placedon the beam path of the output beam 2080 such that the full output beam2080 is received by the power measuring device 2053.

With continued reference to FIG. 20 , the polarization control systemmay further comprise a processing unit 2060 and a control unit 2070. Thepower measuring device 2053 measures the power of the reference beam2081 and subsequently generates a power signal. The generated powersignal may be sent to the processing unit 2060 which processes thereceived power signal (e.g., in a pre-defined manner). Then, theprocessed power signal may be sent to the control unit 2070 which, basedon the processed power signal, generates a corresponding control signal.The control unit 2070 may comprise a memory unit for storage of theprocessed data. Finally, the control signal may be used to make suitableadjustment to the HWP 2029. Note that the processing unit 2060 and thecontrol unit 2070 do not have to be separate units but instead can beintegrated into one processing and control unit (not shown) whichperforms all the tasks performed by the processing unit 2060 and thecontrol unit 2070.

As mentioned above, the polarization control system can be used tooptimize the polarization direction of the pump laser beam with respectto a desired axis of the HC-PCF in at least two scenarios, e.g.,component replacement and long term polarization stability. Thepolarization control system may be operated in two different routines.In the scenario where a key component is replaced, e.g., a gas cell2040, the polarization direction of the pump laser beam 2011 should bere-optimized so as to match a desired axis of the new fiber 2041. Insome embodiments, this may be achieved by rotating the HWP 2029 by onefull turn or 360°. In other embodiments, the HWP 2029 may be rotated bymore than 360°, such as 540°, 720°, 900° or 1080°. In some embodiments,the HWP 2029 may be mounted on e.g., a motorized rotational mount whichrotates the HWP 2029 in small increments. Depending on the resolutionrequirement, each incremental step may correspond to a small rotationangle, such as 1° per step, 2° per step, 5° per step or 10° per step.

With reference to FIG. 20 , during the optimization process, the controlunit 2070 sends a control signal to the HWP 2029 so as to command it toconduct a rotation task, e.g., to incrementally rotate 360° with a stepsize of 5° (or 72 steps). After each rotation step, the power of thereference beam 2081 is measured by the power measuring device, e.g., apower meter. The power signal generated by the power measuring device2053 is processed by the processing unit 2060, e.g., by averaging and/orfiltering. The processed power signal may be sent to the control unit2070 where the power value is saved against the current angular positionof the HWP 2029 e.g., in a memory unit. At the end of the rotation task,i.e. a rotation of 360° with a step size of 5°, a table of data with 72pairs of power and angle values will be generated and stored. Based onthe data, the relation between the power of the output beam 2080 (or thereference beam 2081) and the angular position of the HWP 2029 can bedetermined via, e.g., interpolation of the data. By reference to anexisting PER-power correlation curve, which indicates that each powerminimal corresponds to a PER maximal, the power-angle relation can beconverted to a PER-angle relation from which the optimal HWP positionthat is able to give a maximum PER (or a minimum power) can bedetermined.

In the scenario where long term PER stability is to be monitored and/ormaintained, the power of the output beam 2080 or the reference beam 2081may be either continuously or intermittently measured/sampled by thepower measuring device 2053. Note that it is assumed that thepower-angle relation of the broadband radiation source is alreadyavailable before the long term PER stability can be monitored and/ormaintained. The power signal generated by the power measuring device2053 may be processed, e.g., averaged over a number of sampling points,by the processing unit 2060 before being sent to the control unit 2070.Upon receiving the processed power signal, the control unit 2070 maycompare the processed power signal with a pre-defined power threshold.If the power signal is higher than the power threshold, the control unit2070 may not generate any control signal. However, if the power signalis lower than the power threshold, a control signal is generated. Thecontrol signal may command the HWP 2029 to make a corresponding rotationtask. In some embodiments, the rotation task may comprise a full 360°rotation of the HWP such as the example described above. Alternatively,in other embodiments, the rotation task may comprise a small anglerotation around the current angular position of the HWP 2029. A newangular position of the HWP 2029 may be determined when the PER(manifested by the measured power) of the output beam 2080 or thereference beam 2081 is optimized. The control unit 2070 may then updatethe set angular position with the new angular position in the memory (ifavailable). Following that, the polarization control system continue tomonitor the long term PER stability.

As the power of the output beam 2080 or the reference beam 2081 may beindicative of mode purity, the methods of this section may be used inthe mode control systems described herein.

Note that the number, location and type of the power measuring device2053 are not restricted to what have been described. In someembodiments, a ‘miniature’ power measuring device may be placed in thevicinity of the output end of the HC-PCF fiber such that the power ofthe scattered radiation at the fiber tip can be measured. Since thepower of the scattered radiation at the fiber tip is linearlyproportional to the power of the broadband output beam, theabove-described method for PER optimization still holds valid. In someembodiments, the miniature device may comprise a multiple mode fiber anda power meter. One or both of gas cell 2040 or HC-PCF 2041 may bemounted on a motorized rotation stage and the HWP 2029 may have a fixedangular position. In such a manner, instead of, or in addition to,rotating the HWP 2029 for PER optimization, the gas cell 2040 or theHC-PCF 2041 or both may be rotated to minimize the angular differencebetween the pump polarization direction and one axis of the fiber andthus maximize the PER of the output beam 2080.

It can be appreciated that the method of any of the foregoingembodiments may be used independently of or in combination with any ofthe other embodiments disclosed above, in the latter case as part of themode control system and/or such that any of the foregoing embodiment isused for an initial alignment (coarse and/or fine). Additionally oralternatively the other embodiments may be used subsequently (i.e., whenthe radiation source is properly aligned) for subsequent polarizationoptimization. When used independently, the above-described embodimentsis not restricted to be applied to HC-PCF based broadband radiationsource but can be used to determine and/or optimize a relative anglebetween the polarization direction of a pump laser beam and an opticalplane of a nominally cylindrically shaped waveguide.

It should be noted that the configuration of HC-PCF based broadbandradiation sources is not restricted to the specific arrangementsillustrated or described, and different configurations may beimplemented. For example, the pump laser 1010, 1110, 1210, 1310, 1410may be configured to output a convergent pump laser beam, instead of acollimated pump laser beam 1011, 1111, 1211, 1311, 1411 the waist ofwhich has a good mode matching with the fiber core. In this case, thefocusing lens 1030, 1130, 1230, 1330, 1430 is not required. The beamdelivery system may differ from the specific example of two or moresteering mirrors 1020, 1120, 1220, 1320, 1420, 1021, 1121, 1221, 1321,1421 illustrated. Alternatively, or in addition, at least one of thesemirrors may comprise a curved surface and/or another focusing beamdelivery component provided, which has a radius of curvature (ROC)carefully chosen to form a mode-matched pump spot without the use of thefocusing lens 1030, 1130, 1230, 1330, 1430. According to an embodiment,the input optical window 1042, 1142, 1242, 1342, 1442 may be replaced bythe focusing lens 1030, 1130, 1230, 1330, 1430 and/or the output opticalwindow 1043, 1143, 1243, 1343, 1443 may be replaced by the collimatinglens 1031, 1131, 1231, 1331, 1431. In such a configuration, thedistances between the two lenses and the two fiber ends are chosen suchthat mode matching conditions are well maintained. A broadband radiationsource configured in this way is more compact but less flexible. Inanother embodiment, the gas cell 1040, 1140, 1240, 1340, 1440 mayconsist of multiple sub-cells; and the HC-PCF 1041, 1141, 1241, 1341,1441 may be partially or completely comprised in the sub-cells.Beamsplitter 1051, 1151, 1251, 1351, 1451 may be located outside of therespective detection unit 1050, 1150, 1250, 1350, 1450 rather thaninside as illustrated. The processing unit 1060, 1160, 1260, 1360, 1460and control unit 1070, 1170, 1270, 1370, 1470 are not necessarilyseparate entities and may comprise a single unit with single processorto carry out the processing and control functions.

Although any one of the above-mentioned embodiments is sufficient toindependently perform mode optimization for a broadband radiationsource, some embodiments may be complementary to each other and hencemay be combined to improve the overall performance of the mode controlsystem. For example, in an embodiment, the beam shape measuring device1253 may be added to or combined with the detection unit 1050(comprising bandpass filter 1052 and a power measuring device 1053). Insuch a way, the transverse mode profile of the broadband output beam canbe directly monitored by the beam measuring device 1253 while an outputspectral parameter (e.g., PSD) is measured by the power measuring device1053. Since the power and/or spectral profile of the broadband radiationsource may gradually degrade over time (e.g., caused by aging of pumpdiodes), the spectral parameter (e.g. PSD) values may change (e.g.,drop) correspondingly even though the mode purity remains unchanged.Therefore, the addition of the beam measuring device allows the modecontrol system to quickly validate whether any changed spectralparameter values are caused by the mode degradation or from componentaging induced power degradation and therefore prevent the mode controlsystem from entering an optimization dead-loop if it is the latter.Furthermore, a regular update of reference spectral parameter values isdesirable to reflect component aging induced power degradation. The useof the beam measuring device 1053 helps ensure that the referencespectral parameter values can be regularly maintained and updated whenthe mode purity is optimal.

As such, any of two or more of the detection units 1050, 1150, 1250,1350, 1450 or components therein may be used in combination, eitherdetecting and evaluating the same reference beam or separate referencebeams (e.g., as generated by multiple beamsplitters on the output beam).

Further embodiments are disclosed in the subsequent numbered clauses:

1. A mode control system, being configured for controlling an outputmode of a broadband radiation source comprising a photonic crystal fiber(PCF), the mode control system comprising:

at least one detection unit configured to measure one or more parametersof radiation emitted from the broadband radiation source to generatemeasurement data; and

a processing unit configured to evaluate mode purity of the radiationemitted from the broadband radiation source, from the measurement data,

wherein, based on the evaluation, the mode control system is configuredto generate a control signal for optimization of one or more pumpcoupling conditions of the broadband radiation source, the one or morepump coupling conditions relating to the coupling of a pump laser beamwith respect to a fiber core of the photonic crystal fiber.

2. The mode control system as defined in clause 1, wherein the one ormore parameters of the output radiation comprises one or more parametersindicative of mode purity of the broadband radiation source.

3. The mode control system as defined in clause 1 or clause 2, whereinthe radiation emitted from the broadband radiation source detected bythe detection unit comprises output radiation emitted from an output endof the photonic crystal fiber.

4. The mode control system as defined in clause 3, comprising abeamsplitter located to split a reference beam from a main output beamemitted by the photonic crystal fiber, the output radiation detected bythe detection unit comprising the reference beam.

5. The mode control system as defined in any of clauses 1 to 4, whereinthe at least one detection unit comprises a spectrum measuringarrangement operable to measure one or more spectral parameter values ofthe output radiation as the measurement data.

6. The mode control system as defined in clause 5, wherein the spectralparameter values comprise one or more parameters of a measured spectrumof the output radiation.

7. The mode control system as defined in clause 5 or clause 6, whereinthe spectrum measuring arrangement comprises a spectrum measuring deviceand a multimode optical fiber operable to guide the at least part of theradiation emitted from the broadband radiation source to the spectrummeasuring device.8. The mode control system as defined in any of clauses 1 to 7, whereinthe detection unit comprises one or more bandpass filters, each of theone or more bandpass filters operable to select a respective spectralrange of the output radiation and an illumination measuring deviceoperable to detect an illumination parameter indicative of power of thefiltered radiation, the measurement data comprising and/or being derivedfrom the illumination parameter indicative of power.9. The mode control system as defined in clause 5 or clause 6, whereinthe measurement data comprises a power spectral density or energyspectral density value in one or more spectral ranges, derived from theillumination parameter indicative of power.10. The mode control system as defined in any of clauses 1 to 9, whereinthe detection unit comprises a spatial filter operable to filter outhigher order modes other than a fundamental mode from output radiationand an illumination measuring device operable to detect an illuminationparameter indicative of power of the filtered radiation, the measurementdata comprising and/or being derived from the illumination parameterindicative of power.11. The mode control system as defined in clause 10, wherein the spatialfilter comprises a single mode fiber or a pinhole.12. The mode control system as defined in any of clauses 1 to 11,further comprising a variable polarization arrangement operable tocontrollably configure a polarization property of the radiation emittedfrom the broadband radiation source; wherein the detection unitcomprises an illumination measuring device operable to detect anillumination parameter indicative of power of the radiation emitted fromthe broadband radiation source as a function of the input polarizationangle, the measurement data comprising and/or being derived from theillumination parameter indicative of power; and wherein the mode controlsystem is further operable to optimize the one or more pump couplingconditions by varying the polarization angle imposed by the variablepolarizer with respect to a fiber axis of the photonic crystal fiber,thereby optimizing the pump polarization conditions of the radiationemitted from the broadband radiation source.13. The mode control system as defined in clause 12, wherein thevariable polarization arrangement comprises:

a variable polarizer operable to vary the polarization angle of the pumplaser beam with respect to the photonic crystal fiber, and/or

an actuator operable to vary the angle of the photonic crystal fiberaround its optical axis.

14. The mode control system as defined in any of clauses 1 to 13,wherein the detection unit comprises a beam shape and/or size measuringdevice operable to measure one or more beam characteristics of theoutput radiation related to the shape and/or size of the beam, themeasurement data comprising and/or being derived from the beamcharacteristics of the output radiation related to the shape and/or sizeof the beam.15. The mode control system as defined in clause 14, wherein the beamcharacteristics of the output radiation related to the shape and/or sizeof the beam comprise one or more selected from: beam ellipticity, beamdiameter, Laguerre-Gaussian mode shape, or Zernike polynomial shape.16. The mode control system as defined in any of clauses 1 to 15,configured such that the one or more parameters of radiation emittedfrom the broadband radiation source measured by the detection unitcomprises leakage radiation emitted from the fiber cladding of thephotonic crystal fiber.17. The mode control system as defined in any of clauses 1 to 16,comprising one or more actuators to actuate movement of one or morecomponents of the broadband radiation source; wherein the control signalis operable to control one or more of the actuators.18. The mode control system as defined in clause 17, further comprisinga control unit configured to receive the control signal from theprocessing unit and to control the one or more actuators.19. The mode control system as defined in clause 17 or clause 18,wherein the one or more actuators are operable to optimization of one ormore pump coupling conditions by optimizing one or more selected from:

angular offset of the pump laser beam with respect to the fiber core ofthe photonic crystal fiber;

lateral offset of the pump laser beam with respect to the fiber core ofthe photonic crystal fiber;

beam diameter of the pump laser beam

absolute polarization angle; and/or

divergence of the pump laser beam.

20. The mode control system as defined in any of clauses 17 to 19,wherein the one or more actuators comprise one or more selected from:

at least one actuator for at least one beam steering component or asupport thereof;

at least one actuator for a gas cell of the photonic crystal fiber or asupport thereof; and/or

at least one actuator for a focusing lens for focusing a pump laser beamonto a fiber core of the photonic crystal fiber.

21. The mode control system as defined in any of clauses 1 to 20,wherein the processing unit is operable to evaluate measurement data bycomparing each of the one or more parameters of radiation emitted fromthe broadband radiation source to an equivalent threshold parametervalue indicative of optimal or acceptable mode purity.22. The mode control system as defined in any of clauses 1 to 21,wherein the photonic crystal fiber comprises a hollow-core photoniccrystal fiber (HC-PCF).23. The mode control system as defined in any of clauses 1 to 22,wherein the output radiation of the broadband radiation source comprisesa wavelength range of 200 nm to 2000 nm, or a sub-range within thisrange.24. The mode control system as defined in any of clauses 1 to 23,wherein mode purity describes a ratio between the power in thefundamental transverse mode and the total output power.25. The mode control system as defined in any of clauses 1 to 24,wherein the mode control system being configured to generate a controlsignal for optimization of one or more pump coupling conditions of thebroadband radiation source comprises: being configured to generate acontrol signal for optimization of the one or more pump couplingconditions, so as to maximize mode purity.26. The mode control system as defined in any of clauses 1 to 25,comprising at least one detector for detecting leakage radiation andbeing operable in an initial coarse pump coupling operation to coarselycouple the pump laser beam with respect to the fiber core of thephotonic crystal fiber, the coarse pump coupling operation comprising:

measuring leakage radiation emitted from the fiber cladding of thephotonic crystal fiber during a scanning of the pump laser beam on aninput facet of the photonic crystal fiber; and

determining whether the pump laser beam is coarsely aligned with thephotonic crystal fiber based on the measured leakage radiation.

27. The mode control system as defined in clause 26, wherein determiningwhether the pump laser beam is coarsely aligned comprises locating aminimum value in the measured leakage radiation between at least twomaximum values in the measured leakage radiation.28. The mode control system as defined in clause 26 or clause 27,wherein determining whether the pump laser beam is coarsely alignedcomprises locating a minimum value in the measured leakage radiationinside of a surrounding annular region of maximum values in the measuredleakage radiation.29. The mode control system as defined in any of clauses 26 to 28,wherein the at least one detector comprises a plurality of detectorsspaced radially around the fiber cladding.30. The mode control system as defined in clause 29, wherein eachdetector extends in a radial direction of the photonic crystal fiberabout a longitudinal axis of the photonic crystal fiber over an anglesmaller than 360/n degrees, where n is the amount of detectors31. The mode control system as defined in any of clauses 26 to 28,wherein the at least one detector comprises at least one pair of adetector and mirror, each detector and mirror located at radiallyopposed locations around the fiber cladding.32. The mode control system as defined in any of clauses 26 to 31,wherein the scanning of the pump laser beam comprises a scan along aspiral path on the input facet.33. The mode control system as defined in any of clauses 1 to 32,further comprising an optical manipulation unit for scanning theradiation beam, the optical manipulation unit comprising:

one or more optical elements configured to manipulate the radiationbeam;

a radiation beam tilt adjuster to adjust a propagation direction of theradiation beam; and

a control unit to control the radiation beam tilt adjuster in order toset the propagation direction towards the photonic crystal fiber.

34. The mode control system as defined in clause 35 wherein theradiation beam tilt adjuster comprises at least two wedge prismsarranged in series and a tilt actuation system, and wherein the tiltactuation system is controllable by the control unit to independentlyrotate the wedge prisms about their respective optical axis.35. The mode control system as defined in clause 33 or clause 34,wherein the optical manipulation unit further comprises a radiation beamdisplacement device to displace the radiation beam, wherein theradiation beam displacement device comprises a plane-parallel plate anda displacement actuation system controllable by the control unit torotate the plane-parallel plate about a first axis substantiallyperpendicular to the propagation direction of the radiation beam,wherein desirably the displacement actuation system is furthercontrollable to rotate the plane-parallel plate about a second axissubstantially perpendicular to the propagation direction of theradiation beam and the first axis, and wherein desirably the radiationbeam displacement device comprises a second plane-parallel plate and thedisplacement actuation system is controllable by the control unit torotate the second plane-parallel plate about a second axis substantiallyperpendicular to the propagation direction of the radiation beam and thefirst axis.36. A timing control system, being configured for determining timing ofa broadband radiation source comprising a photonic crystal fiber (PCF),the timing control system comprising at least one pressure sensorconfigured to detect pressure change in a gas environment surroundingthe photonic crystal fiber, and output at least one electrical signal independence of the pressure change, wherein the at least one electricalsignal is used to determine timing of at least one pulse of thebroadband radiation source.37. The timing control system as defined in clause 36, wherein the atleast one pulse of the broadband radiation source comprises a broadbandpulse generated within the photonic crystal fiber and causing thepressure change.38. The timing control system as defined in clause 36, wherein the atleast one pulse of the broadband radiation source comprises a pump pulseconfigured to couple into the photonic crystal fiber and cause thepressure change.39. The timing control system as defined in any of clauses 36 to 38,wherein the photonic crystal fiber comprises a hollow-core photoniccrystal fiber (HC-PCF).40. The timing control system as defined in any of clauses 36 to 39,wherein the at least one electrical signal is configured as a triggersignal for at least one selected from: the pump pulse or the broadbandpulse.41. A broadband radiation source device comprising:

the mode control system as defined in any of clauses 1 to 35; and/or

the timing control system as defined in any of clauses 36 to 40.

42. A metrology device comprising the broadband radiation source deviceas defined in clause 41.

43. The metrology device as defined in clause 42, comprising ascatterometer metrology apparatus, a level sensor or an alignmentsensor.

44. A method of mode control of a broadband radiation source comprisinga photonic crystal fiber, the method comprising:

measuring one or more parameters of radiation emitted from the broadbandradiation source to obtain measurement data;

evaluating mode purity of the radiation emitted from the broadbandradiation source, from the measurement data; and

generating a control signal to optimize of one or more pump couplingconditions of the broadband radiation source, the one or more pumpcoupling conditions relating to the coupling of a pump laser beam withrespect to a fiber core of the photonic crystal fiber.

45. The method as defined in clause 44, wherein the one or moreparameters of the output radiation comprises one or more parametersindicative of mode purity of the broadband radiation source.

46. The method as defined in clause 44 or clause 45, wherein theradiation emitted from the broadband radiation source detected by thedetection unit comprises output radiation emitted from an output end ofthe photonic crystal fiber.

47. The method as defined in clause 46, comprising splitting a referencebeam from a main output beam emitted by the photonic crystal fiber, themeasuring one or more parameters from the reference beam.

48. The method as defined in any of clauses 44 to 47, wherein themeasuring step comprises measuring one or more spectral parameter valuesof the output radiation to obtain the measurement data.

49. The method as defined in clause 48, wherein the one or more spectralparameter values comprise a power spectral density value in one or morespectral ranges.

50. The method as defined in clause 48 or clause 49, comprising bandpassfiltering the radiation emitted from the broadband radiation source andmeasuring an illumination parameter indicative of the power of thefiltered radiation.

51. The method as defined in any of clauses 44 to 50, comprisingspatially filtering out higher order modes other than a fundamental modefrom the radiation emitted from the broadband radiation source andmeasuring an illumination parameter indicative of the power of thefiltered radiation.52. The method as defined in any of clauses 44 to 51, wherein themeasuring step comprises measuring one or more beam characteristics ofthe output radiation related to the shape and/or size of the beam toobtain the measurement data.53. The method as defined in clause 52, wherein the one or more beamcharacteristics of the output radiation related to the shape and/or sizeof the beam comprise one or more selected from: beam ellipticity, beamdiameter, Laguerre-Gaussian mode shape, or Zernike polynomial shape.54. The method as defined in any of clauses 44 to 53, wherein themeasuring step comprises measuring leakage radiation emitted from thefiber cladding of the photonic crystal fiber to obtain the measurementdata.55. The method as defined in any of clauses 44 to 54, comprisingactuating movement of one or more components of the broadband radiationsource based on the control signal so as to optimize of one or more pumpcoupling conditions of the broadband radiation source.56. The method as defined in clause 55, wherein the one or moreactuators are operable to optimize one or more pump coupling conditionsby optimizing one or more selected from:

angular offset of the pump laser beam with respect to the fiber core ofthe photonic crystal fiber;

lateral offset of the pump laser beam with respect to the fiber core ofthe photonic crystal fiber;

beam diameter of the pump laser beam;

absolute polarization angle; and/or

divergence of the pump laser beam.

57. The method as defined in clause 55 or clause 56, wherein the one ormore actuators comprise one or more selected from:

at least one actuator for at least one beam steering component or asupport thereof;

at least one actuator for a gas cell of the photonic crystal fiber or asupport thereof; and/or

at least one actuator for a focusing lens for focusing a pump laser beamonto a fiber core of the photonic crystal fiber.

58. The method as defined in any of clauses 44 to 57, wherein theevaluation step comprises comparing each of the one or more parametersof radiation emitted from the broadband radiation source to anequivalent threshold parameter value indicative of optimal or acceptablemode purity.59. The method as defined in any of clauses 44 to 58, wherein thephotonic crystal fiber comprises a hollow-core photonic crystal fiber(HC-PCF).60. The method as defined in any of clauses 44 to 59, wherein modepurity describes a ratio between the power in the fundamental transversemode and the total output power.61. The method as defined in any of clauses 44 to 60, wherein thegenerating a control signal for optimization of one or more pumpcoupling conditions, optimizes the one or more pump coupling conditionsto maximize mode purity.62. The method as defined in any of clauses 44 to 61, comprising:

an initial coarse pump coupling step to coarsely couple the pump laserbeam with respect to the fiber core of the photonic crystal fiber, thecoarse pump coupling step comprising measuring leakage radiation emittedfrom the fiber cladding of the photonic crystal fiber during a scanningof the pump laser beam on an input facet of the photonic crystal fiber;and

determining whether the pump laser beam is coarsely aligned with thephotonic crystal fiber based on the measured leakage radiation.

63. The method as defined in clause 62, wherein determining whether thepump laser beam is coarsely aligned comprises locating a minimum valuein the measured leakage radiation between at least two maximum values inthe measured leakage radiation.

64. The method as defined in clause 62 or clause 63, wherein determiningwhether the pump laser beam is coarsely aligned comprises locating aminimum value in the measured leakage radiation inside of a surroundingannular region of maximum values in the measured leakage radiation.65. The method as defined in any of clauses 62 to 64, wherein thescanning of the pump laser beam the radiation beam comprises a scanalong a spiral path on the input facet.66. A method of performing a coarse pump coupling step to coarselycouple a pump laser beam with respect to a fiber core of a photoniccrystal fiber, the coarse pump coupling step comprising:

measuring leakage radiation emitted from a fiber cladding of thephotonic crystal fiber during a scanning of the pump laser beam on aninput facet of the photonic crystal fiber; and

determining whether the pump laser beam is coarsely aligned with thephotonic crystal fiber based on the measured leakage radiation.

67. The method as defined in clause 66, wherein determining whether thepump laser beam is coarsely aligned comprises locating a minimum valuein the measured leakage radiation between at least two maximum values inthe measured leakage radiation.

68. The method as defined in clause 66 or clause 67, wherein determiningwhether the pump laser beam is coarsely aligned comprises locating aminimum value in the measured leakage radiation inside of a surroundingannular region of maximum values in the measured leakage radiation.69. The method as defined in any of clauses 66 to 68, wherein thescanning of the pump laser beam the radiation beam comprises a scanalong a spiral path on the input facet.70. A method for determining an optimized value for a polarizationparameter describing an angle of polarization of radiation with respectto an optical plane of a waveguide, the method comprising:

obtaining a relationship between the angle of polarization and a powerparameter indicative of power of radiation having traversed thewaveguide;

obtaining a value for the power parameter, and

determining the optimized value for a polarization parameter from thevalue for the power parameter and the relationship.

71. The method as defined in clause 70, wherein the waveguide comprisesa photonic crystal fiber or hollow-core photonic crystal fiber.

72. The method as defined in clause 70 or clause 71, wherein thedetermining step comprises determining the optimized value as thatcorresponding to a minima for the power parameter.

73. The method as defined in any of clauses 70 to 72, comprising:

varying a polarization of the radiation between a plurality of angularpositions;

obtaining a plurality of values for the power parameter, each valuecorresponding to one of the angular positions; and

selecting an optimized value for the polarization parameter as thatcorresponding to at least one minimum value out of the plurality ofvalues for the power parameter.

74. The method as defined in clause 73, wherein the varying thepolarization of the radiation comprises rotating any one or moreselected from: a polarizer device, a radiation source emitting theradiation, or the nominally cylindrically shaped waveguide.

75. A polarization control system, being configured for controlling theoutput polarization of a broadband radiation source comprising awaveguide, the polarization control system comprising:

at least one detection unit configured to measure one or more parametersof radiation emitted from the broadband radiation source to generatemeasurement data; and

a processing unit configured to infer a polarization property of theradiation emitted from the broadband radiation source from themeasurement data,

wherein, based on the evaluation, the polarization control system isconfigured to generate a control signal for optimization of one or morepump polarization conditions of the broadband radiation source, the oneor more pump polarization conditions relating to the coupling of a pumplaser beam with respect to a fiber axis of the photonic crystal fiber.

76. The polarization control system as defined in clause 75, wherein thewaveguide comprises a photonic crystal fiber or hollow-core photoniccrystal fiber.

77. The polarization control system as defined in clause 75 or clause 76comprising a variable polarizer device for varying the pump polarizationconditions of the broadband radiation source with respect to the fiberaxis of the photonic crystal fiber.

78. The polarization control system as defined in any of clauses 75 to77, wherein the polarizer device comprises a rotatable half-wave plate.

79. The polarization control system as defined in any of clauses 75 to78, wherein the detection unit comprises a power measuring device andthe one or more parameters of radiation comprises power.

80. The polarization control system as defined in any of clauses 75 or79, comprising an optical filter to select one or more desiredwavelengths prior to the detection unit.

81. The polarization control system as defined in any of clauses 75 to80, comprising a measurement branch for collecting a portion of theradiation emitted from the broadband radiation source.

82. An assembly comprising:

an optical element for receiving and modifying radiation;

a receiving element for receiving the modified radiation;

a gas environment for enclosing the receiving element; and

a controlling element configured to stabilize a matching conditionbetween the optical element and the receiving element by adjustingeither the modifying of the received radiation or adjusting a distancebetween the optical element and the receiving element in dependency to aproperty of the gas environment.

83. The assembly of clause 82, wherein the radiation is substantiallymonochromatic, the optical element is a focusing element configured toprovide focused monochromatic radiation to the element and wherein theelement is a non-linear element configured for converting the focusedand monochromatic radiation into broadband radiation.84. The assembly of clause 83, wherein the non-linear element is anon-linear fiber embedded in the gas environment.85. The assembly of clause 84, wherein the property of the gasenvironment is a temperature, pressure or composition of the gas.86. The assembly of clause 85, wherein the controlling element comprisesan actuator configured to variably position the focusing elementrelative to the non-linear fiber along an optical axis of the non-linearfiber.87. The assembly of clause 85, wherein the controlling element comprisesan actuator configured to variably adjust the optical power of thefocusing element.88. The assembly of any of clauses 84 to 87, wherein the matchingcondition is associated with the efficiency of coupling the focusedmonochromatic radiation into the non-linear fiber.89. The assembly of any of clauses 85 to 87, wherein the controllingelement further comprises a processor configured to receive measurementvalues associated with the temperature, pressure or composition of thegas.90. The assembly of clause 89, wherein the processor is configured todetermine a change in refractive index of the gas.91. The assembly of clause 90, wherein the processor is furtherconfigured to determine a focal position variation based on knowledge ofoptical elements positioned in the radiation path upstream of thenon-linear fiber.92. The assembly of any of clauses 89 to 91, further comprising a sensorconfigured to measure the property of the gas environment.93. An assembly comprising:

an optical element for receiving and modifying radiation;

a receiving element for receiving the modified radiation; and

a controlling element configured to stabilize a matching conditionbetween the optical element and the receiving element by adjustingeither the modifying of the received radiation or adjusting a distancebetween the optical element and the receiving element in dependency to aproperty of a gas environment enclosing the receiving element.

94. An assembly comprising:

an optical element for receiving and modifying radiation;

a receiving element for receiving the modified radiation; and

a controlling element configured to stabilize a matching conditionbetween the optical element and the receiving element by adjustingeither the modifying of the received radiation or adjusting a distancebetween the optical element and the receiving element based on anoptical power determined at an exit of the receiving element.

95. The assembly of clause 94, wherein the adjusting is further based onan optical power determined at an entrance of the receiving element.

96. The assembly of clause 95, wherein the adjusting is based on a ratiobetween i) the optical power determined at the exit of the receivingelement and ii) the optical power determined at an entrance of thereceiving element.

Although all the above-mentioned examples and embodiments are inconnection with HC-PCF based broadband radiation sources, an embodimentis equally suitable for mode control of a SC-PCF based broadbandradiation source. In a different embodiment, the detection unit of amode control system measures one or more parameters of the broadbandoutput beam of a SC-PCF based broadband radiation source. Such one ormore parameters should be capable of indicating performance of outputmode of the SC-PCF based broadband radiation source. The measured datais processed in a processing unit and the processed data is subsequentlyevaluated. Depending on the outcome of the evaluation, a feedback signal(or a control signal) is generated and sent to the control unit of themode control system. Finally, the control unit receives the controlsignal and controls one or more active components of the SC-PCF basedbroadband radiation source such that one or more pump couplingconditions of the radiation source are improved and the output modepurity of the SC-PCF based radiation source is improved or optimized.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. Possible other applications include the manufactureof integrated optical systems, guidance and detection patterns formagnetic domain memories, flat-panel displays, liquid-crystal displays(LCDs), thin-film magnetic heads, etc.

Although specific reference may be made in this text to embodiments ofthe invention in the context of a lithographic apparatus, embodiments ofthe invention may be used in other apparatus. Embodiments of theinvention may form part of a mask inspection apparatus, a metrologyapparatus, or any apparatus that measures or processes an object such asa wafer (or other substrate) or mask (or other patterning device). Theseapparatus may be generally referred to as lithographic tools. Such alithographic tool may use vacuum conditions or ambient (non-vacuum)conditions.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention, where the context allows, is notlimited to optical lithography and may be used in other applications,for example imprint lithography.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The descriptions above are intended to beillustrative, not limiting. Thus it will be apparent to one skilled inthe art that modifications may be made to the invention as describedwithout departing from the scope of the claims set out below.

The invention claimed is:
 1. A method of performing a pump coupling tocouple a pump laser beam with respect to a fiber core of a photoniccrystal fiber, the method comprising: measuring leakage radiationemitted from a location on an outer surface of a fiber cladding of thephotonic crystal fiber during a scanning of the pump laser beam on aninput facet of the photonic crystal fiber, the measuring using adetector with a radiation-receiving detection surface facing toward thelocation on the outer surface of the fiber cladding and spaced apartfrom the cladding; and determining whether the pump laser beam isaligned with the photonic crystal fiber based on the measured leakageradiation.
 2. The method of claim 1, wherein the determining whether thepump laser beam is aligned comprises locating a minimum value in themeasured leakage radiation between at least two maximum values in themeasured leakage radiation.
 3. The method of claim 1, wherein thedetermining whether the pump laser beam is aligned comprises locating aminimum value in the measured leakage radiation inside of a surroundingannular region of maximum values in the measured leakage radiation. 4.The method of claim 1, wherein the scanning of the pump laser beamcomprises a scan along a spiral path on the input facet.
 5. The methodof claim 1, wherein the determining is a coarse alignment and furthercomprising performing a fine alignment using the pump laser beam at apower level higher than as used in the coarse alignment.
 6. The methodof claim 1, wherein the measuring uses a plurality of detectors spacedaround the fiber cladding.
 7. The method of claim 6, wherein eachdetector of the plurality of detectors extends in a radial direction ofthe photonic crystal fiber about a longitudinal axis of the photoniccrystal fiber over an angle smaller than 360/n degrees, where n is theamount of detectors.
 8. The method of claim 1, wherein the measuringuses at least one pair of a detector and mirror, the detector and mirrorof the pair located at radially opposed locations around the fibercladding.
 9. A system for performing a pump coupling step to couple apump laser beam with respect to a fiber core of a photonic crystalfiber, the system comprising: a detector configured to measure leakageradiation emitted from a location on an outer surface of a fibercladding of the photonic crystal fiber during a scanning of the pumplaser beam on an input facet of the photonic crystal fiber, wherein thedetector has a radiation-receiving detection surface facing toward thelocation on the outer surface of the fiber cladding and spaced apartfrom the cladding; and instructions, when executed by a processor orcontroller, configured to determine whether the pump laser beam isaligned with the photonic crystal fiber based on the measured leakageradiation.
 10. The system of claim 9, wherein the instructionsconfigured determine whether the pump laser beam is aligned are furtherconfigured to locate a minimum value in the measured leakage radiationbetween at least two maximum values in the measured leakage radiation.11. The system of claim 9, wherein the instructions configured determinewhether the pump laser beam is aligned are further configured to locatea minimum value in the measured leakage radiation inside of asurrounding annular region of maximum values in the measured leakageradiation.
 12. The system of claim 9, wherein the scanning of the pumplaser beam comprises a scan along a spiral path on the input facet. 13.The system of claim 9, wherein the detector comprises a plurality ofdetectors spaced around the fiber cladding and wherein each detector ofthe plurality of detectors extends in a radial direction of the photoniccrystal fiber about a longitudinal axis of the photonic crystal fiberover an angle smaller than 360/n degrees, where n is the amount ofdetectors.
 14. The system of claim 9, wherein the detector comprises atleast one pair of a detector and mirror, the detector and mirror of thepair located at radially opposed locations around the fiber cladding.15. The system of claim 9, wherein the determination of whether the pumplaser beam is aligned is a coarse alignment and the instructions arefurther configured to cause performance of a fine alignment using thepump laser beam at a power level higher than as used in the coarsealignment.
 16. A broadband radiation source device comprising: a sourceof radiation; and the system of claim
 9. 17. A metrology devicecomprising: the broadband radiation source device of claim 16; and afurther detector.
 18. A mode control system comprising: a detectorconfigured to measure leakage radiation emitted from a fiber cladding ofa photonic crystal fiber during a scanning of a pump laser beam on aninput facet of the photonic crystal fiber; and instructions, whenexecuted by a processor or controller, configured to evaluate modepurity of the radiation emitted from a broadband radiation sourcecomprising the photonic crystal fiber, from the measured leakageradiation, wherein, based on the evaluation, the mode control system isconfigured to generate a control signal for optimization of one or morepump coupling conditions of the broadband radiation source, the one ormore pump coupling conditions relating to the coupling of the pump laserbeam with respect to a fiber core of the photonic crystal fiber.
 19. Abroadband radiation source device comprising: a source of radiation; andthe system of claim
 18. 20. A metrology device comprising: the broadbandradiation source device of claim 19; and a further detector.